agent: user-facing text through i18n keys, a hardcoded string is found too late

[landsman]

The llm-i18n branch mixed three things: the localisation guidance, a make fmt
target with a pre-commit hook, and a 16-file markdown rewrap. This is the
guidance alone.

Restructured to match the layout from #74 — a rule file under
shared/.claude/rules/ with a row in the index — instead of landing in this
repo's own AGENTS.md, which is about working in this repo, not about building
applications. Rewritten in the same voice as the other rules: the reason first,
the bullet soup cut down, British spelling like behaviour in
commit-messages.md.

Unconditional, no paths: frontmatter: the trigger is writing a user-facing
string, not touching a particular file, and the default it overrides — type the
English in — happens before any path is known.

Left on llm-i18n: make fmt and the pre-commit hook. Separate topic, and the
mass rewrap is coupled to them — without it the first commit touching any
markdown file rewraps the whole thing. Say the word and that goes up as its own
PR.