feat(app): add reasoning effort to the composer

[iamnbutler]

Closes #146.

• Wire the shared effort picker beside model selection using channel model capabilities.
• Persist effort in pi agent configuration, with Default/Off semantics for always-thinking models.
• Reject unsupported levels and active-run changes; serialize concurrent admissions per chat.
• Reset composer effort to model default when switching models.
• Fix the unused effort menu’s missing Base UI group wrapper, which otherwise crashes the UI when opened.

Validation

• bun types, bun run ci, and production app build pass.
• Real GPT-5.4 mini runs at Low and Off completed; Low persisted after reopen; unsupported Max and concurrent busy-run changes were rejected.
• Native Ace-dev smoke: effort is visible beside the model picker; menu lists Off, Low, Medium, High, Extra high; selecting High closes the menu and updates the composer label. Native snapshots verified both states.
• Full GUI-send smoke was not completed: AX value replacement was accepted but had no visible effect in the composer (existing native editor input gap Focus and paste into WebKit composers with native desktop tools desktop-tools#5). Real provider admission/persistence was verified separately above.

Built in Ace

Entirely in Ace using the channel, lane, shell, review subagent, and native desktop tools. The dev runner was stopped/restarted through Ace’s shell after the menu crash because native background Cmd+Q cannot be verified and the app has no reload command (development-loop gap #55). No external editor or agent used.