[flexdinesh]

Fixes / opening the wrong worktree after diffx . in a linked worktree.

Cause: local scope widens as each checkout finishes collecting (4 parallel workers). / opened the most active head among whichever checkouts had finished first, then never replaced it. That's often a small, older worktree.

• Behavior owner and resulting change:
  • serverapp.Published.Newest: changed heads first, then the deepest checkout whose root contains the launch path (current request's TriggerRoot), then activity. Uses the request metadata because reused observations keep their first run's metadata.
  • POST /api/v2/collections returns defaultContextId. / waits for local startup collection (existing skeleton), then opens it. /streams/{id} still opens immediately.
  • collector discovery canonicalizes symlinked input paths so they compare against Git's canonical checkout roots.
• Contracts/invariants affected; local/remote implications:
  • CollectionResult adds the required field defaultContextId (local-only route). Bare /mcp default follows the same rule. Remote is unaffected.
  • Clean launch checkout falls back to the most active changed head.
• Validation results; remaining risks:
  • mise run check: pass (Go, contracts, 91 browser tests).
  • New tests: Go launch preference (incl. reused observation, nested worktree, name-prefix sibling), symlinked input; browser root waits for default (verified failing without the fix).
  • Real-binary smoke: diffx . in a linked worktree returned that worktree as default over a more active main checkout.
  • Risk: fresh / loads now wait for full local collection instead of the first finished checkout.
• Intentional architecture/design changes and rationale: docs/system.md, docs/development.md, usage.md updated. The launch path expresses which checkout the user is working in.
• UI evidence: no visual changes; reuses the existing collecting skeleton.