Preserve session content when switching executors

[bborn]

Summary

• When switching executors on an existing task (e.g. Claude → Codex), the previous session's conversation history is now extracted and included as context in the new executor's prompt
• Adds GetSessionContent(workDir) to the TaskExecutor interface, implemented for all 6 executors (Claude, Codex, Gemini, Pi, OpenClaw, OpenCode)
• Works in both daemon (background execution) and interactive (TUI) paths
• Large sessions are truncated to ~50K chars, keeping the most recent context

How it works

1. When starting a new executor session, GetPreviousSessionContent() checks if any other executor has session files for the same workDir
2. If found, it reads and formats the conversation as a "Previous Session Context" section prepended to the prompt
3. Session file parsing supports: Claude/Pi JSONL, Codex/Gemini JSON (with generic message extraction for varying formats)
4. Detection is automatic — if the current executor already has its own session, no handoff is needed

Test plan

• Unit tests for JSONL parsing (Claude-style messages, empty files, system-only messages)
• Unit tests for JSON parsing (role/content, sender/text, nested content arrays, Gemini parts, auto-discovery)
• Unit tests for truncation behavior and handoff formatting
• Unit tests for Claude config dir integration
• All existing executor tests pass
• All UI tests pass
• go vet clean

🤖 Generated with Claude Code