Add scheduled recurring agent runs

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Description

Let a project start agent sessions on a schedule: a fixed prompt, at a fixed time, in a fresh worktree.

Pane already knows how to create a session from a prompt — this makes that repeatable without a person present. Nightly bug sweeps, a weekly dependency check, a "review yesterday's diffs" pass every morning: the work that is worth doing regularly but never worth remembering.

A new Scheduled runs entry in the project's menu manages them.

What it does

• Three schedule shapes rather than cron syntax: every N minutes, daily at HH:MM, weekly on a weekday at HH:MM. Each one can be stated in a sentence, which is what the dialog shows back to you (Every day at 03:00 — next run in 4h 12m). Cron is more expressive than anyone actually needs here and impossible to render as a sentence.
• Each run creates a real session, exactly as the create dialog would: prompt, agent (claude, or none for a plain terminal) and an optional worktree/branch template.
• Per-run state is visible: last run, its outcome, the error if it failed, and a link to the session it created.
• Enable/disable without deleting, and Run now — which starts one immediately without moving the cadence, so testing a schedule at 15:00 does not shift the nightly run.
• Missed runs are skipped, not caught up. Waking a laptop after a weekend must not start three days of nightly sweeps at once; a run more than 15 minutes late is recorded as skipped and the schedule moves on.
• Deleting a project deletes its schedules (ON DELETE CASCADE).

How it works

• scheduleCalculator.ts is pure and holds every decision about when: computeNextRun, isDue, isMissed. Time zones, daylight saving and "the next weekly run when today already passed" are the parts that break silently, so they are unit-tested directly rather than through the scheduler.
• ScheduleManager ticks every 30s and asks the calculator; it talks to storage through a small ScheduleStore interface, so its behaviour is tested without a database.
• Sessions are created through the existing task queue with createSessionAndWait, so lastSessionId is the real session id rather than a queue job id — the difference only shows up when you click the link.
• Persistence is one table, scheduled_runs, plus an index on (enabled, next_run_at_ms) so the tick is a single indexed lookup.
• Channels are daemon-owned (schedules:*): the schedule belongs to the machine that runs the agents.

Type of Change

• New feature (non-breaking change which adds functionality)

Checklist

• I have read the CONTRIBUTING.md guidelines
• My code follows the code style of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes
• I have run pnpm typecheck and pnpm lint locally
• I have tested the Electron app locally with pnpm electron-dev

Critical Areas Modified

• State management/IPC events

Additional Notes

Schema note: schema.sql is split on the statement separator at startup, so a ; inside a comment breaks the database. The new block is written accordingly.

Tested in the running app, not only in unit tests: a schedule was created, fired on its own tick, produced a real session in a fresh worktree, recorded its outcome, and was then disabled and deleted. Missed-run handling was exercised by moving a schedule's due time into the past.