fix(web): expose shared file actions on composer mentions

[saphid]

What Changed

Composer file mentions lacked the shared file menu available from file lists and diffs. Pointer context-menu and keyboard context-menu commands now invoke the same shared actions using the composer environment and worktree. The existing preview click and editor content model remain.

Why

Implements the proposed invariant in pingdotgg#12683. This PR is stacked on that exact documentation branch in the fork; each invariant pair is independent.

Both plain and rich modes use Tiptap. This independent change covers supported workspace-relative file mentions; absolute/UNC resolver extensions and chat-chip parity remain owned by pingdotgg#11859. Captured attachments, directories, skills and unresolved mentions are not substituted with live files. No React Native file-menu change.

Verification

45 tests across composer mention identity, shared file-menu policy and mention parsing passed; web typecheck and targeted lint passed (existing warnings).

Independent review: direct SWE-2 Max via Devin, high reasoning requested, source-only review exited 0. Conditional findings were checked against the owning source; no unresolved actionable findings for this final change. The initial tool-based attempt returned no verdict after read commands required confirmation.

UI Changes

Runtime proof (2026-09-20, isolated proof app; base d6f2913 vs this branch): right-clicking a composer file-mention chip on base shows no menu; on this branch the chip opens the shared file actions (Open / Reveal in Finder / Open with).

Before: https://github.com/user-attachments/assets/dfffa1e2-1cff-4392-ad1f-ddaece813844
After: https://github.com/user-attachments/assets/d9c56f81-4cad-4403-a942-4096ee8834fd

Method note: the contextmenu was dispatched via DOM evaluation (the automation click tool has no secondary button); the chip also handles ContextMenu / Shift+F10. Native surfaces unchanged; T3 Device tools remain unavailable.

Tracking: https://github.com/saphid/personal-ops/issues/142

Model: GPT-6 Astra via Codex/T3 Code.

[saphid]

Superseded by pingdotgg#12869 — the implementation PR now lives upstream against main (it contains the docs commit until the paired docs PR merges). Same head branch, same commits; this fork draft is closed to avoid a duplicate review surface.