Persistent checkpoint & session history

[hellodk]

Description

Implement persistent storage for checkpoints and session history to survive VS Code restarts. Currently, checkpoints (shadow-copy snapshots before each agent action) and chat session data are held in memory and lost when VS Code closes.

Business Value

• Users can restore previous states even after restarting VS Code
• Session history remains accessible across multiple work sessions
• Enables audit trails and compliance use cases
• Improves user experience by preventing loss of work context

Acceptance Criteria

• Checkpoints are automatically serialized to disk in a directory
• Each checkpoint file is versioned with timestamp and includes file paths, before/after diffs, and metadata
• Chat session history is persisted to with one JSON file per session
• Sessions are indexed and searchable by date, workspace, and content keywords
• Restore Checkpoint command reads from disk and presents UI to select historical checkpoints
• Cleanup Old Sessions command removes checkpoints and sessions older than N days (configurable, default 30)
• Existing in-memory checkpoint API remains compatible — storage layer is transparent to calling code
• Storage size is bounded — implement LRU eviction if checkpoint dir exceeds configured limit (default 500MB)
• Sealed Secrets pattern: checkpoint paths are always workspace-scoped (reject path traversal)
• New config settings: champ.storage.checkpointRetentionDays, champ.storage.maxCheckpointSizeMB, champ.storage.sessionsPath

Tests Required

• Unit: checkpoint serialization (create, read, delete from disk)
• Unit: session history JSON schema validation
• Unit: checkpoint file naming and versioning
• Unit: LRU eviction algorithm (size bounds)
• Unit: path traversal protection in checkpoint paths
• Integration: write checkpoint to disk, restart VS Code, verify restore works
• Integration: session history persists across multiple chat sessions
• Integration: cleanup command correctly removes old checkpoints based on age
• E2E: user can see checkpoint history UI, select one, and restore state

Implementation Notes

• Use VS Code's workspace storage API ( for sensitive data) and filesystem for large blobs
• Store relative paths in checkpoint metadata — convert to absolute on restore to support moved workspaces
• Checkpoint storage format: {timestamp}-{hash}.json with gzipped file diffs to save space
• Session format: {sessionId}.json with streaming-friendly structure (append messages, not rewrite)
• Consider using SQLite or LevelDB for efficient querying across old sessions (future optimization)

Related Issues

None yet — this is a foundational feature.

Environment

• VS Code 1.95+
• All platforms (Windows, macOS, Linux)