feat(agents): a failed resume opens the agent's own session picker (#311)

[GabrielDumbrava]

Closes #311

Problem

A stored session id that no longer resolves exits within RESUME_FAILURE_MS, and termic answered it with a fresh session and a toast.
The conversation was usually still there: only termic's pointer was stale.
The agent's own reason for refusing ("No conversation found", or claude's "running as a background session ... claude attach") was repainted over by the respawn.

What this does

A new per-agent capability, resume_picker_args, names the args that open the agent's own session picker, as discussed on the issue: no termic-side reading of any agent's session files.

• When a stored id fails to resume, the next spawn opens that picker instead of minting a fresh session.
• The session picked there comes back over the hooks (session <id>), exactly as any /resume does since claude tab resumes the session from before /clear after a relaunch #306, so the next relaunch resumes it.
• Leaving the picker (Esc exits 1) starts a fresh session, once; there is no loop back into the picker.
• The toast offers the picker and carries the agent's own line, read from the raw output of the resume's first RESUME_FAILURE_MS (src/lib/resumeTail.ts). xterm's buffer can lag the exit event; Rust drains every byte before emitting it.
• Without agent hooks the pick cannot be reported, so the toast says to install them. The picker still gets the user their conversation now.
• An agent with no picker keeps today's fresh fallback, with the agent's line in the toast.

The field is editable in Settings > Agents ("Session picker args") and a clone inherits its parent's.
There used to be a "Resume it" banner for this; it outlived the failure it described, which is why this is the agent's own picker plus a transient toast, and nothing termic keeps on screen.

Defaults: only what is measured

Only claude is installed here, so only claude gets a default: --resume with no id.
Measured on claude 2.1.278:

Action	Result
claude --resume --name <task>	opens claude's "Resume session" picker (search, this project's sessions)
pick a session	SessionStart with source: resume, the chosen id, entrypoint cli, which the READY hook reports
Esc	exits 1, no SessionStart

Every other agent stays empty until measured, per docs/adding-an-agent.md, which now lists what to measure (the picker opens with termic's name_args after it, a pick is REPORTED, leaving it exits). The rest are for the follow-up PR.

Existing installs get claude's default through the same load-time backfill as the other capability lists, which is safe since #316.

Review fixes (second commit)

A read-only review by a second agent found four things, each checked in the code:

• The pty-exit listener is now registered before pty_attached. A process that has already exited has its buffered output released by the ack, and the waiter fires pty-exit right behind it; acking first left the exit to race onPtyExit's own round trip. Pre-existing for every fast exit, but a lost exit here is a failed resume that neither opens the picker nor starts fresh. No test forces this race; it is a timing window.
• The resume-tail capture checks one boolean on the data path, closed by the existing survive timer, instead of a time comparison per chunk for the life of every resumed session.
• The hookless case and its toast, above.
• The Settings hint no longer promises that an emptied picker field stays empty on a built-in agent: the backfill restores it, as for the other lists.

Tests

• npm test: 2276 passed. New: spawnArgsForCli with a picker (replaces every resume block, keeps --name), resumePickerArgsForCli (built-in default, none for an unmeasured agent, clone inheritance and override), and resumeTail (escape and OSC stripping, line joining, trimming).
• cargo test: 1076 passed. New: an existing claude entry without the field gains --resume on load; codex stays empty.
• agent.e2e.ts, new describe (3 cases), with fakeclaude reproducing claude's three shapes (a dead id prints "No conversation found" and exits 1; --resume alone is the picker; a lone Esc leaves it): a failed resume spawns --resume with no id instead of --session-id, the toast offers the picker and carries the agent's line, the picked id is stored and resumed on the next relaunch, and leaving the picker starts fresh exactly once. Controls: without the picker all three fail; without the fallback the last one does. Passed 3 runs in a row.
• Full e2e suite: the same 11 failures in the same 8 spec files as a clean main build (85b4d12); none only on this branch.

Manual verification

Not manually verified in the app: testing it here would have cost me a live session, so I opened this without a hand test.
The claude behaviour it relies on (picker opens, a pick fires SessionStart source=resume, Esc exits 1) was measured directly against claude 2.1.278 in a scratch directory.

🤖 Generated with Claude Code

[simion]

Merged, thanks @GabrielDumbrava. Race reorder is correct and the picker loop avoidance holds up, only a minor cosmetic nit noted, nothing blocking.