feat(tasks): persist unsent prompt drafts across restarts

[johannesjo]

Problem

Text typed into a task's Send a prompt box lived only in a component-local signal. Quitting the app or restarting the machine discarded it. Task notes were already persisted, so the two boxes behaved inconsistently.

Fix

The prompt draft now follows the same path notes already take. A promptDraft field on the task is mirrored from the prompt box on every keystroke:

• src/store/tasks.ts gains a setTaskPromptDraft setter. Empty text is stored as undefined, so the saved file does not gain a promptDraft: "" key on every task.
• src/components/PromptInput.tsx wraps its text setter to write through to the store, and hydrates the textarea from the saved draft at mount. Every existing caller of that setter persists too, including the "send notes to prompt" button and coordinator prefills.
• src/store/persistence.ts serializes the field and restores it on both the active and collapsed task paths, ignoring a non-string value from a corrupt file.
• src/store/autosave.ts tracks the field, so a draft on its own triggers a save.

Saving reuses the existing autosave: a 1s trailing debounce capped at 5s during continuous typing, plus a flush on window close. A graceful quit or reboot loses nothing, and a hard power cut loses at most the last few seconds. That matches how notes already behave.

Tests

Eight new tests cover the round trip: save, restore for active and collapsed tasks, corrupt-value rejection, the autosave snapshot, and four component tests for hydration, write-through, and clearing on send.

Full suite passes at 2410 unit tests and 179 client tests, plus compile, typecheck, lint at zero warnings, and Prettier.

Notes for the reviewer

• The transient promptDraftActive flag is deliberately still not persisted. It only gates coordinator hand-back, and the coordinator auto-fire path reads the live textarea contents rather than that flag, so a restored draft still suppresses auto-fire correctly.
• Document-workspace agent tasks are not in taskOrder, so their drafts are not written to disk. Those tasks are themselves ephemeral and never persisted, so this is consistent rather than a gap.
• One behavior change worth a look: a restored draft that differs from a still-pending initialPrompt will block that prompt's auto-send. The same guard already applied before a restart, so this preserves an already-blocked state rather than creating a new one.

https://claude.ai/code/session_011Mkrn85hgdP6oXb6F57s3j