feat: add Let's discuss button to post-outline plan approval

[Nathan Schram (nathanschram)]

Summary

• Adds a "Let's discuss" button to the post-outline plan approval keyboard (between Approve Plan/Deny and cancel)
• When clicked, tells Claude Code to ask the user what they'd like to discuss about the plan before deciding
• New chat action in claude_control.py with _CHAT_DENY_MESSAGE
• Handles both da: prefix (synthetic) and hold-open (real request_id) paths
• Clears cooldown and outline_pending state on both paths
• Post-outline keyboard now has 2 rows: [Approve Plan | Deny], [Let's discuss]

Test plan

• 5 new tests added, 5 updated for new button layout
• 1773 tests pass, 81% coverage
• Ruff format + lint clean
• Lockfile in sync

🤖 Generated with Claude Code

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