Remove unused branch metadata and split_diff_view from sessions

[rumpl]

Remove BranchParentSessionID, BranchParentPosition, and BranchCreatedAt fields from the Session struct and Summary struct. These were stored and persisted but never read for any logic or UI display.

The branching mechanism itself (BranchSession) remains functional - only the provenance metadata that nothing consumed is removed.

Also drops the split_diff_view column which was already ignored in code.

Adds migration 019 to drop the columns and index from SQLite.

[docker-agent]

AI Code Review

Assessment: 🟡 NEEDS ATTENTION

Summary

This PR removes unused branch metadata fields and the split_diff_view column from the Session and Summary structs, along with their database storage. The code changes are well-structured and maintain consistency across both InMemory and SQLite stores.

Key Changes:

• ✅ Removed unused fields from Session and Summary structs
• ✅ Updated all SQL queries to exclude removed columns
• ✅ Added migration 019 to drop columns and indexes
• ✅ Test coverage updated to match new behavior

Issue Found:

• ⚠️ MEDIUM severity: The migration uses SQLite's DROP COLUMN syntax which requires SQLite 3.35.0+ (March 2021). On systems with older SQLite versions, the migration will fail and prevent the application from starting.

Recommendation

Consider adding a SQLite version check before executing the DROP COLUMN statements, or use a table recreation pattern for broader compatibility:

-- Create new table without old columns
CREATE TABLE sessions_new AS SELECT id, tools_approved, ... FROM sessions;
DROP TABLE sessions;
ALTER TABLE sessions_new RENAME TO sessions;
-- Recreate indexes

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1 finding • Generated by docker-agent AI reviewer