feat: add skill british-english

Adds a British English writing-conventions skill covering -ise/-our/-re/-ence/-ogue spellings, doubled-l verb forms, programme vs program, punctuation (single quotes, logical punctuation, spaced en dashes, Oxford comma), vocabulary, dates/times, grammar (collective nouns, shall/will, prepositions, present perfect), measurements and currency. Polishes a few entries on second pass: doubled-l wording, mediaeval note, en-dash example, and 'informal' label.

british-english/SKILL.md

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+---
+name: british-english
+description: Apply British English spelling and grammar as a global default whenever a project does not explicitly declare its language in CLAUDE.md or AGENT.md. Invoke when creating or editing files, writing commit messages, generating documentation, or producing any user-facing text in a project that has no project-level language override. Covers -ise endings, -our/-re/-ogue spellings, doubled-l verb forms, noun/verb pairs such as practise/license, and other British–American differences.
+---
+
+# British English
+
+Conventions for writing in British English, as distinct from American English. Follow these rules consistently throughout a document.
+
+## Spelling
+
+### -ise not -ize
+Use -ise endings (the dominant British convention outside Oxford University Press):
+
+organise, realise, recognise, apologise, specialise, summarise, emphasise, authorise, categorise, prioritise, criticise, customise, finalise, minimise, optimise, standardise, sympathise, visualise
+
+**Exception:** -yse is always British (never -yze): analyse, paralyse, breathalyse, catalyse
+
+### -our not -or
+colour, favour, honour, humour, labour, behaviour, neighbour, glamour, odour, rigour, valour, flavour, harbour, vapour, savour, endeavour
+
+### -re not -er
+centre, fibre, theatre, metre, litre, lustre, sombre, sabre, calibre, manoeuvre, spectre, reconnoitre
+
+### -ence/-ence not -ense
+defence, offence, pretence, licence (noun) / license (verb), practice (noun) / practise (verb)
+
+**Remember:** noun = c, verb = s. "The doctor's **practice**. She **practises** medicine. A driving **licence**. You are **licensed** to drive."
+
+### -ogue not -og
+catalogue, dialogue, monologue, analogue, prologue, epilogue, travelogue
+
+### Doubled consonants
+British doubles the final consonant before suffixes regardless of stress:
+
+travelled, travelling, traveller, cancelled, cancelling, modelled, modelling, labelled, labelling, counselled, counselling, marvellous, signalling, worshipped, jewellery, fulfilled, skilful, wilful, enrol, instalment
+
+### Programme vs program
+- **programme** = broadcast, event, plan, schedule
+- **program** = computer software only
+
+### Other British spellings
+aluminium, aeroplane, grey, tyre, mould, plough, sulphur, pyjamas, cheque (bank), draught (beer/air), kerb (road edge), storey (of a building), sceptic, artefact, mediaeval (older British form; medieval is now standard in modern usage), annexe (noun), furore
+
+## Punctuation
+
+### Quotation marks
+- **Single quotes** for primary quotations: 'like this'
+- **Double quotes** for quotes within quotes: 'He said "hello" and left'
+
+### Logical punctuation
+Place commas and full stops **outside** quotation marks unless they are part of the quoted material:
+
+- The minister called the proposal 'absurd'.
+- She asked, 'What time is it?'  (question mark is part of the quote)
+- 'I shall return,' he said.  (comma is part of the quoted speech)
+
+### Oxford comma
+Optional in British English. Generally **omit** unless needed to prevent ambiguity:
+
+- "red, white and blue" (standard)
+- "my parents, the Queen, and the Prime Minister" (needed to avoid implying your parents are the Queen and PM)
+
+### Dashes
+Use spaced en dashes for parenthetical statements:
+- "The policy – introduced last year – has been revised." (note the spaces around the en dash)
+
+## Vocabulary
+
+| British | American |
+|---|---|
+| boot | trunk |
+| bonnet | hood |
+| windscreen | windshield |
+| lorry | truck |
+| petrol | gas/gasoline |
+| motorway | freeway/highway |
+| pavement | sidewalk |
+| car park | parking lot |
+| number plate | license plate |
+| flat | apartment |
+| ground floor | first floor |
+| first floor | second floor |
+| lift | elevator |
+| torch | flashlight |
+| wardrobe | closet |
+| cooker | stove |
+| bin | trash can |
+| rubbish | garbage/trash |
+| garden | yard |
+| post | mail |
+| postbox | mailbox |
+| postcode | zip code |
+| mobile (phone) | cell phone |
+| maths | math |
+| full stop | period |
+| university | college (informal) |
+| term | semester |
+| head teacher | principal |
+| marks | grades |
+| revision | review (studying) |
+| biscuit | cookie |
+| crisps | potato chips |
+| chips | French fries |
+| aubergine | eggplant |
+| courgette | zucchini |
+| coriander | cilantro |
+| sweets | candy |
+| fizzy drink | soda |
+| takeaway | takeout |
+| queue | line |
+| holiday | vacation |
+| fortnight | two weeks |
+| chemist | drugstore |
+| shop | store |
+| bill | check (restaurant) |
+| nappy | diaper |
+| dummy | pacifier |
+| autumn | fall |
+| aluminium | aluminum |
+| tyre | tire |
+| grey | gray |
+
+## Dates and Times
+
+- **Day-Month-Year:** 15 March 2026 or 15/03/2026
+- No comma between month and year
+- 24-hour clock in formal writing: 14:30
+- 12-hour clock with lowercase: 2.30pm (note: full stop not colon, no space before am/pm)
+
+## Grammar
+
+### Collective nouns as plural
+When emphasising the individuals in a group, use plural verbs:
+- "The government **are** divided on this issue."
+- "The team **are** playing well."
+- "The committee **have** decided."
+
+Singular is also acceptable when the group acts as a unit: "The company **is** based in London."
+
+### Got not gotten
+- "I've **got** a new car." (possession — standard British)
+- "She's **got** better at it." (become — standard British)
+- Never use "gotten" — it's American.
+
+### Shall vs will
+- **Shall** for first-person offers and suggestions: "Shall I open the window?" "Shall we go?"
+- **Will** for simple future: "I will be there at three."
+- In formal/legal writing, "shall" indicates obligation (though "must" is increasingly preferred).
+
+### Past participles — irregular forms preferred
+learnt, dreamt, burnt, spoilt, smelt, spelt, knelt, leant (not learned, dreamed, burned, etc. — though both accepted)
+
+### Prepositions
+- "at the weekend" (not "on the weekend")
+- "in hospital" (not "in the hospital")
+- "at university" (not "in college")
+- "different from" or "different to" (not "different than")
+- "fill in a form" (not "fill out")
+- "write to someone" (not "write someone")
+
+### Present perfect for recent events
+- "I've just eaten." (not "I just ate.")
+- "Have you seen it?" (not "Did you see it?" — for recent events)
+
+### Needn't
+"You needn't worry." is standard British alongside "You don't need to worry."
+
+## Measurements
+
+Metric is official, but imperial persists in specific contexts:
+- **Miles** for road distances and speed (mph)
+- **Feet and inches** for human height
+- **Stones and pounds** for body weight (1 stone = 14 pounds)
+- **Pints** for beer, cider, and milk
+- **Celsius** for temperature
+- **Kilograms** for other weights
+- **Litres** for fuel (but miles per gallon for economy)
+- **A4 paper** (210 x 297mm), not Letter size
+
+## Currency
+- Pound sign before the figure: £50, £1,200
+- Pence with p: 50p
+- No space between symbol and number
+- "Per cent" as two words when spelled out; % symbol with no space: 10%
+
+## Authoritative Sources
+
+- New Oxford Style Manual (Oxford University Press)
+- Guardian and Observer Style Guide
+- Cambridge Guide to English Usage
+- Fowler's Modern English Usage
+- GOV.UK Style Guide