[Juliusolsson05]

Fixes #1178

Problem

A terminal's only "last active" time was runtime.terminalForeground.changedAt. It lived in memory and was stamped now on the first foreground observation after any attach. Every restart pulls a foreground snapshot into empty runtimes, so every terminal read "active just now" after a restart, and Close Old Agents could never find a shell nobody had touched in days. A command that finishes inside one poll (1 s) and plain typing never counted at all.

What changes

• SessionMeta.lastUsedAt (terminals) is persisted with the workspace, so it survives restarts.
• It moves only on real use:
  • typing or pasting in TerminalLeaf, the one input path for plain shells;
  • a real foreground transition (a command started or finished, a cd).
  • The first observation after an attach is never use.
• A terminal with no record gets a floor on first sighting: "not used since at least now". Without it, a shell nobody touches after this ships would stay "unknown activity" forever, and Close Old Agents skips unknown rows.
• Writes are throttled to one a minute per terminal, so typing doesn't autosave on every key. Close Old Agents' finest unit is minutes.
• Readers: Close Old Agents and Agent Activity both use the record. A parked shell with no runtime now ages too, instead of reading unknown.
• changedAt keeps its other jobs: marking the pane unread when a command finishes, and the live status.

Review round (one round: Codex, Pi)

Fixed:

• A terminal nobody is observing yet counts as possibly running. After a restart, a parked tmux shell is not polled until it is woken, so it could be running a dev server. It now has an age, so without this Close Old Agents would have listed it as idle. The row says "not observed yet", and it is excluded unless Include running is ticked. (Codex, major)
• Ages read "record + 1 minute", a true upper bound on the last use. The throttle drops uses within a minute of the record, so the bare record let a one-minute threshold call a shell used a second ago old. (Codex, major)
• Use is stamped from the user's own keydown, IME and paste events, not xterm's onData:
  • onData also carries automatic terminal replies, which counted as use (Codex, minor);
  • its buffered branch meant typing while a shell was waking never counted (Codex, major).
• Undo Close keeps the record (Codex, minor).
• The reader is renamed terminalLastUsedUpperBound for what it returns (Pi).

Accepted gap (Pi): dictating into a plain shell does not count as use until the dictated command runs (which the foreground change then catches). The dictation hook has no access to the workspace, and adding it isn't worth it here.

Not done, on purpose

tmux session_activity would only cover tmux-backed shells, and it counts output as activity, so a prompt clock or tail -f would keep a forgotten shell "in use" forever.

Verification

• Unit tests for the helper: the throttle, a clock that goes backwards, non-terminals, and a floor that never moves an existing record.
• Hook test: the post-restart snapshot is only a floor, and a real change is a use.
• Close Old Agents tests:
  • a shell unused for 3 days is still found right after a restart;
  • a parked shell with no runtime ages.
• Mutation check: I restored the old reader and the old "first observation = use" rule. The tests failed both times.

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