Fix deleted repo group resurrection bug

[jfversluis]

Problem

When a user deletes a sidebar group that is linked to a tracked repository, the group reappears ("resurrects") on the next UI state refresh. This happens because ReconcileOrganization() enforces the invariant "every tracked repo must have a sidebar group" — but this conflicts with the user's explicit deletion.

Root Cause

Three code paths in ReconcileOrganization call GetOrCreateRepoGroup, which recreates the group if the repo is still tracked by RepoManager:

1. Auto-link sessions (line 214): Session's working directory matches a worktree → creates repo group
2. Reassign orphaned sessions (line 234): Session has WorktreeId but is in _default group → creates repo group
3. Ensure repo coverage (line 259): Tracked repo has no group → creates repo group

Fix

• DeletedRepoGroupRepoIds — new HashSet<string> on OrganizationState (persisted to organization.json) that tracks repo IDs whose groups were explicitly deleted by the user
• GetOrCreateRepoGroup(repoId, name, explicitly) — new explicitly parameter:
  • Implicit calls (from ReconcileOrganization) return null for deleted repos
  • Explicit calls (from repo-add operations) clear the deleted flag and create the group
• DeleteGroup — now clears WorktreeId on moved sessions for repo groups, preventing ReconcileOrganization from reassigning them via the worktree path
• Backward compatibility — null-coalesce guard in LoadOrganization handles old organization.json files without the new field

Tests

13 new tests in DeletedRepoGroupResurrectionTests covering:

• Delete adds to deleted set / non-repo delete doesn't
• WorktreeId cleared for repo groups / preserved for non-repo groups
• Implicit GetOrCreateRepoGroup returns null for deleted repos
• Explicit GetOrCreateRepoGroup clears deleted flag
• Existing group returned even if in deleted set
• ReconcileOrganization doesn't resurrect deleted group
• Session not reassigned after group deletion
• Serialization round-trip of DeletedRepoGroupRepoIds
• Legacy JSON backward compatibility
• Default group deletion is no-op
• Re-add repo clears deleted flag

All 1157 existing tests pass (1 pre-existing locale-dependent failure unrelated).