fix: prevent terminal query response leakage

[linletian]

Summary

• Add terminal connection state machine (ttyState) to prevent query leakage
• Add isTerminalQueryResponse() filter to block xterm.js auto-generated responses
• Fix ttyState reset on instance stop and prevent reconnection for stopped instances
• Document known limitations and future considerations in ARCHITECTURE.md §5.9

Problem

When switching terminal windows or after TUI programs (like opencode CLI) exit, xterm.js sends terminal query responses that appear as garbage text in the shell:

• ;1R (cursor position report fragment)
• rgb:0b0b/1010/2020 (OSC color response fragment)
• ?2027;0$y (DEC private mode report fragment)

Root Cause

Shell sends query → xterm.js receives → generates response → forwards via onData() → sent back to PTY → displayed as garbage

Changes

1. State Machine: IDLE → CONNECTING → HANDSHAKING → READY → DISCONNECTING
2. Focus Prevention: Terminal only gains focus when ttyState === 'READY'
3. Response Filter: isTerminalQueryResponse() blocks known response patterns
4. State Reset: ttyState properly reset when instance stops
5. Reconnection Logic: Only reconnect if instance is still running

Documentation

• Added docs/TERMINAL_FILTER_REVIEW.md with detailed analysis
• Added docs/ARCHITECTURE.md §5.9 with data flow diagram
• Added code comments linking to review document

Known Limitations

⚠️ See docs/TERMINAL_FILTER_REVIEW.md for complete analysis:

• Cannot distinguish user-intentional ESC sequences from auto responses
• Regex may have edge cases or cause false positives
• Small coordinate reports may be incorrectly filtered
• Some response types (DSR, etc.) not explicitly covered
• Filter is in input path; alternative approaches documented

Test Plan

• Verified Connection Lost issue resolved (missing variable declaration fixed)
• Verified terminal focus timing (state machine)
• Verified garbage text reduction after TUI program exit
• Build passes: go build ./...