refactor: remove Mission Control and dead terminal code; key agent columns "agent-<id>"

[Ziinc]

Removes Mission Control and the dead code the sessions chaos QA found. It also includes #636, which was merged into this branch.

Mission Control

• The only way to open it was a two-finger touch swipe (useTwoFingerSwipe). Desktop trackpads send wheel events, not touch events, so it could not be reached.
• Removed:
  • TerminalMissionControl and its test;
  • terminal-mission-control/ (group builder, diff-stats hook, preview formatter, and their tests);
  • useTwoFingerSwipe and its test;
  • the terminal-mission-control screenshot spec;
  • the Mission Control and Gestures sections in web/docs.
• useTerminalSessionSummaries kept a throttled preview of each terminal's output, plus timestamps, only for Mission Control. It now tracks only isStreaming, the one field the sidebar spinner and the idle-agent lookup read.
  • TerminalSessionSummary drops previewOutput, lastUserInputAt and lastActivityAt.
  • isTerminalSessionIdle and TERMINAL_IDLE_THRESHOLD_MS are gone.

Other dead code

• WorkspaceEditSession.tsx and Terminal.tsx: nothing imported them.
• get_window_repo_path: only WorkspaceEditSession called it. Removed the TS wrapper, the Rust command and its registration. The setter stays, because agent_runtime reads the map.
• Pane handle methods with no callers: createAgentSession, focusTerminal, closeTerminal, closeIdleTerminals, closeAllTerminals.
• The agent argument of onCreateNewSession, which no caller passed.
• The workspaceBranchName option of getOrCreateSession, which was never read.

Agent column ids (from #636)

Agent terminal columns were keyed claude-<sessionId>, although every supported agent runs in the same kind of column. They are now agent-<sessionId>. The PTY id (session-<id>) is unchanged.

• agentTerminalId() and agentSessionIdOf() in terminal/agentTerminalId.ts build and parse the id. They replace every hand-built claude- string.
• Renamed the pane identifiers:

  Before	After
  ClaudeSessionData	AgentSessionData
  claudeSessions	agentSessions
  handleCloseClaudeSession	handleCloseAgentSession
  TerminalEntry type "claude"	"agent"

Tests

• Rewrote the summaries hook tests for the streaming flag:
  • process output marks the terminal streaming;
  • local echo does not;
  • an idle pulse clears it without a filesystem refresh;
  • a burst of output publishes once.
• New unit test agentTerminalId.test.ts. Tests and specs select agent columns with [data-terminal-id^="agent-"].
• Checks: tsc, lint, cargo fmt and cargo clippy are clean.
• Tests: unit tests pass. CI integration passed on both commits before refactor(terminal): key agent columns "agent-<id>" instead of "claude-<id>" #636 was merged in.
• Screenshot specs: these fail on main too, because they look for a "New agent terminal" button that no longer exists:
  • workspace-delete-closes-terminals
  • treq-send-review-comments
  • terminal-header-workspace-label
  • terminal-pane-inset-controls

[github-actions]

🚀 Web preview: https://preview-629.treq-9zy.pages.dev
📊 Lighthouse report: https://github.com/treq-dev/treq/actions/runs/36566658200/artifacts/11032731530

[Ziinc]

After merging main in, test-js / test:integration failed on two tests in workspace/create-pr.test.tsx:

• pushes a rewritten (diverged) branch before creating a PR;
• hides Create PR when there is no GitHub remote, whose setup setupPushedWorkspaceWithGitHub reported not_on_remote as true right after a push.

I don't think this PR caused it:

• The PR only removes Mission Control and unused terminal-pane code. It doesn't touch push, remote status or Create PR.
• The same tests passed on this PR's first CI run, before the merge. They also passed on fix(terminal): keep focus on close, relaunch without the task on model change, drop Reset #628, which merged the same main.
• On this head (f6fc7bd3) the file passes 13/13, twice, locally.

I haven't found or made a fix for it. I'm re-running the failed job once. If it fails again I'll treat it as a real failure and dig in.

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