fix(session): reconcile models after branch navigation

[boadij]

Summary

• restore Downshift premium/economy phase from the active session-tree branch
• reconcile model and thinking level after tree navigation and existing-session restoration
• preserve session-wide enablement, premium capture, and safety pause state
• fix stale persisted session IDs after fork or clone

Implementation

• add pure branch-phase restoration
• add position-only target reconciliation
• handle session_tree
• reconcile restored state during session_start
• avoid threshold evaluation and handoff generation during navigation
• keep switch causes separate from desired targets by routing all automated changes through switchToTarget()

Non-goals

• changing the model used for Pi branch-summary generation
• adding branch-aware configuration
• changing the persisted-state schema
• upgrading Pi dependencies

Testing

• npm ci
• npm run check
• npm run release:dry
• pi -e . startup/load smoke test: passed, with the local extension registered successfully
• automated coverage includes session_tree, reload/resume/fork/clone restoration, fresh startup, disabled/off/paused behavior, handoff suppression, internal model-change protection, threshold downshift, compaction upshift, and matching-target no-op behavior

The full interactive /tree, reload, resume, fork, clone, and model-transition matrix was not completed in this terminal session because it requires an interactive Pi session and configured provider credentials/model operations.

Notes

Pi generates optional branch summaries before session_tree, using the source branch's active model. This PR intentionally reconciles only after successful navigation so model-change and Downshift state entries are written to the destination branch.