feat(channels): suggest joined channels in the composer

[wesbillman]

Summary

Add #channel suggestions through the Channels plugin and the existing composer completion host. The plugin owns channel eligibility, filtering, ranking and insertion; the host retains editor transactions, keyboard/IME handling, focus, positioning and stale-result protection. No new public plugin API or relay endpoint.

• Suggest up to 20 confirmed joined stream/forum channels, including private channels. Exclude archived, cached, read-only and unnamed/multiline entries; qualify duplicate names by ID and recheck eligibility/name on selection.
• Insert an escaped, stable-ID Markdown link, not notification recipients. Suppress suggestions inside code, existing links/images and HTML. Empty ready results hide the menu so ordinary PR #1234 + Enter sends.
• Filter the existing local channel list without typing-driven network reads. The syntax matcher examines at most 160 characters; the extra prose-context scan runs only after a channel trigger matches.

Validation

Checked clean head 83ee83921636a9083e74715ee60809d12c6e4f0d, rebased onto 59e87ce5. The feature patch is unchanged from the independently reviewed f6096600 (git range-diff equality).

• bin/pnpm exec vitest run: 451 files, 5,480 tests passed. The earlier run on the old base failed one profile-menu focus assertion; the upstream fix waits for focus, and the complete suite passes after rebase. No unrelated feature-branch fix added.
• Actual pre-push hooks passed: TypeScript, 35 related files / 525 tests, design TypeScript and all design guards. Clippy correctly skipped this JS/CSS-only patch. Biome checked all 10 changed supported files with no warnings; diff check clean.
• bin/pnpm test:browser channel-completion.spec.mjs mentions.spec.mjs --project chromium --project webkit --no-deps: 10/10 passed. Covers real editor selection/undo, focus, portal and narrow-screen geometry, restored drafts, signed channel/thread content without recipient tags, formatted acceptance, no-match sending, and existing mention journeys.
• Independent agent review found no remaining blockers. The initial local slice was also tried successfully by the requesting human.

Test-layer accounting

Added one browser scenario, executed in both engines; removed none. It proves native editing/undo, focus, actual layout and plugin-to-send wiring that jsdom cannot establish. Eligibility, ranking, lifecycle, escaping and prose-context permutations stay in colocated Vitest tests. No tests were moved or replaced. No pre-feature failing browser run was recorded.

Local typing diagnostic

With 1,002 channels and a 10,000-character draft, 48 input-to-next-frame samples per case on the pre-rebase feature tree:

Input	Chromium p95, enabled / disabled	WebKit p95, enabled / disabled
Active #general	23.5 / 19.7 ms	20 / 20 ms
Ordinary text	21.2 / 21.3 ms	23 / 22 ms

Zero added relay reads were observed. These are local diagnostic measurements, not a zero-cost claim or production latency guarantee; they were not repeated after the patch-preserving rebase.

Remaining gates and known limitation

Draft pending a human recheck of the final review-driven behavior changes, especially no-match sending and formatted insertion. The initial human try preceded those adjustments. To check: type #, select with Enter/Tab, undo, repeat in a thread and bold/italic text, then type a nonmatching PR #1234 and verify Enter sends normally. Confirm private-channel labels and narrow-window placement.

Hosted DCO Check passed at 83ee8392; CI is running. CI and required reviewer/code-owner approval remain merge gates. Native desktop build, the entire browser suite and dedicated channel-link insertion while editing a previously sent message were not exercised locally.

Optional follow-up: if refreshing the channel list has failed and a query has no matches, the shared Retry-only menu captures Enter to retry rather than send. Escape dismisses it. This is documented, not silently treated as fixed.

[wesbillman]

No actionable source defects found in channel eligibility, stale-selection protection, escaped link insertion, prose-context gating, or composer lifecycle/focus handling.

Star Lord’s automated source review via Wes’s account; head 83ee83921636a9083e74715ee60809d12c6e4f0d, base 59e87ce5de792737a93d335ea345f7d829bc61ac. I executed no PR code or tests; the hosted snapshot shows CI required and DCO successful, but this is not runtime acceptance or approval. The documented final human recheck, native-desktop acceptance and dedicated previously-sent-message editing check remain outstanding; Retry-only Enter behavior remains the disclosed optional follow-up.