feat: non-blocking Agent and editable AI resume canvas

[avabbbb]

Summary

Applies the remaining OfferU 2026-10 implementation patches on top of current main.

0000-docs-restructure.patch is intentionally excluded: that work is already merged in PR #67. This PR applies the functional work from 0001–0003.

Agent interaction

• keep input, Skill selector and quick actions usable while a Run is active or waiting
• add /runtime/runs/{id}/steer for mid-run guidance
• queue messages that cannot be injected immediately
• park waiting Runs without cancelling or approving them
• switching/new conversations detaches the stream instead of aborting the Run
• preserve newer main-side resume/recovery state while integrating the patch

Resume canvas

• add paper / 「纸 · 可直接编辑」 template
• 1:1 finished resume is the editor; direct text edits write back to source fields
• add/delete bullets and items, Esc rollback, page-overflow marker and paper tone switch
• add inline AI diffs with adopt / modify / skip
• selection toolbar: 更精炼 / 对齐岗位 / 换个说法 / 留言给 AI
• collect multiple comments and send them to Agent in one request
• open Agent rail/panel automatically for composed canvas intents

Conflict semantics

• change proposal conflict detection from whole-resume invalidation to target-section conflict
• edits elsewhere no longer invalidate an unrelated suggestion
• if the exact target section was edited by the user, the suggestion cannot overwrite it; user version wins
• JD/profile fact changes still retain the existing proposal-staleness behavior

Tests added/updated

• Agent never-locks / steer / queue / navigation behavior
• editable Paper template
• inline proposal diff and canvas comment tray
• page-level Paper proposal interactions
• section-level resume proposal conflicts
• Agent compose mailbox

Integration note

The patch was generated against an earlier main. Current main contains newer AgentPanel state (sendRequestPendingRef, resume-candidate/recovery flow). Those newer changes were preserved while integrating the non-blocking interaction behavior.

This PR is intentionally not merged.

[avabbbb]

Fix for PR #73 (feat/agent-resume-canvas-2026-10)

PR #73 at a97a80a fails 4 tests in frontend/src/components/workbench/AgentPanel.test.tsx
(main 5092639's tests conflict with 0001's never-lock semantics):

• allows a new message while an Ask answer remains pending
• keeps the composer editable during a request while preventing duplicate starts
• detaches the stream when the panel closes without cancelling its durable Run (cascade)
• sends the auto routing sentinel for a plain natural-language request (cascade)

This one commit touches only AgentPanel.tsx and AgentPanel.test.tsx:

• pending Ask is not "busy": a new message starts a new turn and parks the old Run (main's rule)
• double-send guard moved into startTurn; a message sent while running is queued and sent exactly once
• main's duplicate-start test now asserts the queue semantics

Verified locally on top of a97a80a: npm run typecheck ✅, npx vitest run ✅ 40 files / 181 tests.