fix(sandbox): re-point a thread at a live GitHub connection when its own is gone

[pedrofrxncx]

Stacked on #5807 (merge that first; this PR's base is its branch).

Problem

threads.metadata.githubRepo.connectionId pins a specific mcp-github connection, but those aren't immortal:

• VIRTUAL_MCP_DELETE tears down the agent's repo-scoped child connection;
• re-importing a repo mints a new connection id.

Either way every thread pinned to the old id is permanently un-startable, even when the org still has a perfectly good connection for the same repository. Prod today: 152 threads reference a connection id that no longer exists (e.g. thrd_sUNCH2u3kuYEdSE4mb5Ca → conn_JLu7_P8GYpTVtsx6sUHQ_).

Change

getThreadGithubRepo — the single read every consumer goes through (sandbox provisioning, fs tools, dispatch) — now, when the pinned connection is missing:

1. looks for a live connection granting the same owner/repo via the existing findReusableRepoConnection (org-shared wins, same rule as import);
2. returns the re-pointed binding and persists it to the thread metadata;
3. falls back to the dead binding untouched when nothing matches, so the user still gets GITHUB_CONNECTION_MISSING (fix(sandbox): tell the user the GitHub connection is gone, not unauthenticated #5807) telling them to link the repo again.

Best-effort: any failure logs and returns the original binding. The choice itself is a pure function, repointedRepoBinding.

Note: the sandbox branch embeds the connection id (threadBranch), so a repoint yields a fresh sandbox — the old one was unusable anyway.

Testing

• 4 new unit tests on repointedRepoBinding (match, org-shared preference, no match, already-live no-op)
• bun run check, bun run fmt

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Summary by cubic

Automatically re-points a thread’s GitHub repo binding to a live connection for the same owner/repo when its pinned connectionId no longer exists, and persists the fix. Falls back to the original binding so users still see GITHUB_CONNECTION_MISSING when no replacement is available.

• Bug Fixes
  • Added repointedRepoBinding to pick a live repo connection (prefers org-shared via findReusableRepoConnection) and update getThreadGithubRepo.
  • Persist the new binding in thread metadata so sandbox, fs tools, and dispatch all use the same live connection; repointing yields a fresh sandbox branch.
  • Best‑effort behavior: on errors or no match, keep the old binding and surface GITHUB_CONNECTION_MISSING.
  • Added 4 unit tests for repointedRepoBinding (match, org-shared preference, no match, already-live no-op).

^{Written for commit b5dc1a5. Summary will update on new commits.}