search: support all result types (checkpoints, commits, sessions) and align with UI

Previously the CLI hardcoded types=checkpoints when calling the search API, so commits and sessions were never returned. This aligns the CLI with the UI by:

• Removing the types=checkpoints filter so the API returns all result types
• Adding CommitResult and SessionResult types with custom JSON marshal/unmarshal
• Adding polymorphic Result accessors (ResultOrg, ResultRepo, ResultTitle, etc.)
• Adding type filter tabs (0-3 keys) and Type column to the TUI
• Adding commit and session detail views
• Adding --all-repos flag (defaults to current repo scoping)
• Aligning default limit to 100 (matching the UI, was 200)
• Including API-provided type counts and timing metadata in responses