fix(QUA-577): recover partial typed input when agent finishes mid-typing

[Desperado]

Summary

Fixes QUA-577. The v1.13 "type while agent runs, auto-queue" feature was silently dropping the user's typed-but-not-yet-Enter-terminated input when the agent's response came back before they could press Enter.

Root cause (from e2e PTY probe)

• StartQueueReader accumulates each typed character in an in-goroutine lineRunes buffer. On Enter the buffer is pushed onto the queue.
• When stopQueueReader() fires (agent turn ended), the goroutine returns and lineRunes is garbage-collected — even if it contained a partial line the user was mid-typing.
• Additionally, bytes that arrive between stopQueueReader() and the next tui.ReadInput() end up split across two prompts.

Reproduction (real keyboard, confirmed by @strazhnyk): submit a slow prompt, start typing a second one, and let the agent finish before you press Enter. Your typed text vanishes; the next prompt opens empty.

Fix

• StartQueueReader now returns func() string. The goroutine sends its final lineRunes value through a buffered channel before exiting; the stop function returns it after wg.Wait.
• repl.go reads the partial after the agent's turn finishes and, if non-empty after trim, pushes it onto the queue with a clear notice:

  ↩ partial input recovered → queued [N]: <text>  (type /queue clear to discard)
  

• New /queue clear slash command (PromptQueue.Clear) lets the user discard recovered text if it was stray input.
• Windows stub updated to match the new signature.

Verification

• e2e PTY harness: pre-fix, the typed prefix was lost and never appeared in the queue. Post-fix, the partial is recovered, queued, and processed on the next iteration. Subsequent characters that arrived during the stopReader→ReadInput window are captured by the new queue reader and queued cleanly too.
• go test ./... -short -count=1 is green.
• New unit tests: TestPromptQueue_ClearReturnsCount, TestPromptQueue_ClearEmpty.

Test plan

• Real-keyboard regression: open qmax-code, submit a slow prompt, type a few chars during thinking but don't press Enter. Confirm the agent's response is followed by ↩ partial input recovered → queued [1]: <your text>, then processing queued prompt.
• Same flow but press Enter mid-thinking: confirm the existing ✓ queued [N]: path still works and Enter still cancels the running agent.
• /queue clear discards queued items as expected.
• No regression in go test ./....

🤖 Generated with Claude Code

[qualitymaxapp]

✅ QualityMax Pipeline

Gate	Result
🔍 AI Review	✅ Clean
🧪 Repo Tests	✅ 287/287 passed (go)
🤖 AI Tests	✅ 28/32 passed

_{Powered by QualityMax — AI-Powered Test Automation}

[Desperado]

Included and merged via #103 (db91e5d). Closing this stale duplicate PR to avoid re-merging the same change.