[Meta] Restore features from old Ace #6

Features the original Ace had that this version doesn't expose yet, and that aren't already tracked in the dogfooding backlog (#5). Plan and Document tabs are deliberately excluded.

Items are ordered by priority. Connect existing shared UI components where they fit.

Port from old Ace

• Project dashboard: pick back up, needs-you, and team pulse #36: project dashboard (pick back up, needs-you, and team pulse). Needs you, Pick back up, and a rule-based Team pulse shipped in feat(app): show needs you, pick back up, and team pulse on the project dashboard #471. Left: linking issues to their channels (needs Start work from issues and PRs #28) and a model-written pulse.
• Surface CI status in channel details and the chat when relevant #29: show CI status in channel details and the chat when relevant.
• Expose Git and PR actions in the UI #20: Git and PR actions in the UI. Create PR, Commit, Update PR, Rebase, Address review, Ready, Merge, and Close shipped in the sidebar and Diff header (feat(app): offer Create PR and fixing failed checks by asking the chat's agent #472, feat(app): offer commit, rebase, update, merge, and PR state actions by asking the chat's agent #475, feat(app): offer git and PR actions in the Diff header, and Close for a PR that can't merge yet #478). Left: verify Address review and Rebase end to end.
• Fork a channel #33: fork a channel into a new sibling channel. Forking local channels shipped in feat: fork a channel into a sibling channel #470 (sidebar menu and ace fork). Left: cloud compute channels, and forking from a message (needs Expose several chats in a channel's UI #19).
• Connect message editing, deletion, and reactions #25: message editing, deletion, and reactions.
• Support non-image uploads in the composer #26: non-image uploads in the composer. Text files (feat: send text files from the composer to the agent #469), PDFs as extracted text and opening files from the timeline (feat(app): send PDFs as their text and open attached files from the timeline #479), and refusing attachments that exceed the model's context (fix(channel): refuse attachments that can't fit the chat's model, and explain context overflows #481, fixes An attachment larger than the model's context breaks every later run in the chat #480) shipped. Left: scanned PDFs and other binaries.
• Connect dictation in the composer #38: dictation in the composer. Dictation shipped in both composers, inserting at the cursor (feat: transcribe dictation into the channel composer #473, feat(ui): dictate into a composer that already has text, and in the dashboard #477). Left: verify in the desktop app, including the microphone and speech permission prompts.

These are tracked in #5 instead: #19, #22, #24, #28, #35. Done or not planned: #21, #23, #27, #30, #31, #32, #34, #37, #59.

Migrated from TODO.md.