fix(coding-agent): route idle extension slash commands

[code-yeongyu]

Summary

• route idle builtin extension slash commands through the extension session prompt path
• keep adjacent slash commands such as /resume on the normal command path
• skip fully malformed empty session logs in the session observer scanner

Root Cause

The interactive submit handler only dispatched extension commands while another response was streaming or compaction was active. When idle, /sessions fell through as regular user input, so the picker never opened.

Validation

• targeted regression suite: npx tsx ../../node_modules/vitest/dist/cli.js --run test/suite/session-observer-scanner.test.ts
• full repository check: npm run check
• manual TUI QA for valid, empty, malformed, and adjacent /resume session states

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Summary by cubic

Fixes idle routing for extension slash commands so /sessions opens the picker even when no response is streaming. Also ignores malformed session logs and keeps adjacent commands like /resume on the normal path.

• Bug Fixes
  • Route idle extension commands via the extension session prompt in InteractiveMode (preserves history and clears input).
  • Skip empty or malformed session logs in the session scanner to avoid blocking the picker.
  • Maintain pass-through for non-extension or adjacent slash commands (e.g., /resume, unknown commands).
  • Added tests for picker open, empty state, malformed files, and command pass-through.

^{Written for commit 896b957. Summary will update on new commits. Review in cubic}