feat(app): populate the channel details sidebar

[iamnbutler]

The details sidebar only showed the summary. It now follows Codex's sidebar and old Ace's session details, adapted to Ace (a channel is not one PR; the sidebar follows the root chat's current lane):

• Changes: +adds -dels for the chat's work; clicking shows the Diff tab, opening one beside the chat if none is open.
• Branch: the checked-out branch, now reported by changes as Changes.branch (optional, so older workers still work).
• Pull request: the newest PR from that branch with its state, plus a checks summary for open/draft PRs. New host op github-pull (gh pr list --head … --json …,statusCheckRollup), local-only like the other GitHub reads. Read while the sidebar is open, again when the branch head moves and every minute; not cached.
• Subagents: the root chat's subagent calls with their task's first line, model, and running/done/failed/stopped state.
• Links: URLs people and agents wrote (not tool output), newest first, GitHub issues/PRs shortened to #N; first five, with Show all.
• Channel: project, host, owner, model, total tokens and estimated cost across chats, created.

Views publish what they already load (info, transcript once live, changes) into a small per-channel store the sidebar reads, so it adds no channel requests. Documented in docs/architecture.md.

Smoke-tested against a source dev host with a channel on PR #122's checkout: the sidebar showed the branch, PR #122 with "Checks passed", links in mention order, and the Changes row opened and then reselected one Diff tab. Not exercised live: subagents and usage (the dev profile has no provider key).

Built in Ace

Code, builds, and the dev host ran in Ace. The UI smoke test drove the dev host's web app with Playwright's cached Chromium because Ace's desktop tools reported "Open Ace on this host" while Canary was open, and screenshots can't be viewed by the agent: #128.