feat(web): add composer file-reference picker

[dylantirandaz]

What this changes

• Adds an @ file-reference picker to the composer. Type @ to open a ranked list of the session's files.
• Accept a directory to list its children. Accept a file to insert its path into the draft.
• Accepted references show as chips above the composer. The draft text is the only state: each chip is a view of the text, and removal edits the text.
• The picker reads the session's file index and loads directories on demand. It closes when the composer is disabled, under the same rules as the slash menu.
• Moves the query-ranking functions from slash.ts into match.ts, so the slash menu and the picker share one ranker.

Implements the file/path-references row of FEATURE_MATRIX §4 (text insertion plus chips; reorder is not included).

Verification

• pnpm check passes: release contract, provenance, lint, typecheck. The provenance record for the extracted scorer now covers slash.ts and match.ts.
• pnpm --filter @t4-code/web test passes: 74 files, 1099 tests. The 20 new tests cover the trigger rules, listing flattening, ranking order, insert spans, and chip tokens.
• Drove the picker in a browser on fixture data: @ opens the menu; ranking puts basename matches first; a directory accept loads its children and keeps the menu open; typed paths load directories on demand; a file accept inserts the path and shows a chip; chip removal edits the draft; Escape closes the menu.

Checklist

• pnpm check passes (release contract, provenance, lint, typecheck)
• pnpm test passes; new behavior has a test that fails without this change (ran the @t4-code/web suite; the known packages/client transcript-retention failure on main is unrelated)
• Any pasted logs or screenshots are redacted: no secrets, tokens, pairing codes, or private paths
• Ported code (T3 Code / OMP) follows the provenance rules in THIRD_PARTY_NOTICES.md

[wolfiesch]

Made a few more quick changes:

• Blocked absolute and parent-directory file queries before they reach the host file-list command.
• Added CJK and emoji filename support.
• Kept email-like text from rendering as a file chip.
• Avoided adding a second space when inserting a reference before existing whitespace.
• Merged current main while preserving transcript export, then reran pnpm check and the full test suite.