fix(session): checkout claimed PR branch

[GautamSharma99]

Summary

• Switch a session workspace to the claimed PR source branch during claim-pr.
• Return branchChanged: true when checkout actually changes the workspace branch.
• Persist updated session branch metadata after a successful checkout.
• Surface typed errors for dirty worktrees, missing branches, invalid branches, and branches checked out elsewhere.

Why

ClaimPR previously persisted PR ownership and facts but always returned BranchChanged: false. The session workspace could
remain on its old AO branch even after claiming an existing PR, which made the CLI/API checkout status misleading and left
the worker in the wrong branch.

Implementation Notes

• Added Workspace.CheckoutBranch.
• Implemented gitworktree checkout via git switch.
• Refuses dirty worktrees before switching branches to avoid carrying unrelated local edits across branches.
• Uses existing branch sentinel errors where possible.

Tests

• go test ./internal/service/session
• go test ./internal/adapters/workspace/gitworktree
• go test ./internal/httpd/controllers
• go test ./internal/session_manager ./internal/integration
• npm run lint
• git diff --check