fix(I2): exempt /clio-matters from the IP-keyed general limiter

The per-user bucket inside the /clio-matters router was decorative: the
app-level generalLimiter still ran in front of it, keyed by IP, at a shared
300 requests per 15 minutes. The pilot firm NATs its whole office through one
address, so that IP bucket - not the per-user one - was the binding constraint
the moment two solicitors browsed matters, and exhausting it would degrade
every other route for their colleagues.

Exemption is a skip on the prefix (kept where generalLimiter is mounted, so the
JSON body parser and everything after it still apply - mount-order would have
skipped those too). Exact-segment match, not a bare startsWith, so a future
`/clio-matters-anything` cannot inherit the exemption.

Corrects the rationale comments, which claimed matters browsing could exhaust
the "shared research allowance" and take /companies and /legislation down:
/clio-matters was never on researchLimiter - it was on generalLimiter, so the
stated mechanism was wrong even before this change. Fixed in
routes/clioMatters.ts, index.ts, .env.example and the CLAUDE.md registry row.

index.ts retains its pre-existing prettier deviation at makeLimiter (the file
was not prettier-clean at baseline); only the hunks added here are normalised.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>