feat: search workspace prompt history with Ctrl+R

[AetherAI3]

Closes #325.

What changed

• Add a bounded, incremental Ctrl+R overlay over the prompt history already loaded for the current workspace. Empty query selects newest; typing filters; repeated Ctrl+R walks older matches without wrapping.
• Enter inserts the complete stored match into the editable composer. Escape restores the exact prior draft and cursor. Search and the slash picker hand ownership back to each other without submitting a task.
• Cap loaded history at 1,000 entries, examine at most 16 KiB per entry and 1 MiB total per query update, and clip only the displayed result. Oversized and budget-skipped entries are counted in the panel.
• Keep search idle-composer-only; busy typeahead, PTY, approval, setup, paste, and non-TTY routes retain their existing input behavior. AETHER_NO_HISTORY disables search contents.

Evidence

• npm run build passed.
• npm run docs:check passed.
• 49 scoped tests passed across history search/store, composer, queue/typeahead, input buffer, and slash picker. A final 16-test console/search run passed after the last integration change.
• Fixtures cover duplicate, multiline and Unicode prompts, no match, disabled history, oversized and clipped entries, workspace isolation, undo/cancel boundaries, picker transitions, busy typeahead, and zero model calls during editing.