fix(web): always run a repo-backed editor on a thread (#6667 follow-up)

[guitavano]

Summary

After #6667 started addressing destinations by path, a repo-backed agent editor could open with no thread in the URL (?thread= absent — a deep link, choose-editor, or any navigation that didn't carry it, since the shell deliberately doesn't retain thread across navigations).

The branch is a thread field (activeTask.branch), so a threadless editor left the branch picker stranded on "Select branch…", with both selecting a branch and New silently doing nothing (setCurrentTaskBranch is gated on an active thread), and the adopt / auto-fresh-stale effects never firing.

AgentInsetProvider now guarantees a thread for repo-backed agents: when the URL names none it mints one into ?thread= before mounting, reusing the user's idle empty "New chat" for that agent when one exists — exactly what useNavigateToAgent does (findReusableNewChat ?? fresh id). This restores the pre-#6667 behavior where the client minted the thread id and the ensure-fallback created the row on landing.

Scoped to repo-backed agents on purpose — the Super Agent (no repo, no branch pill) keeps its lazy threadless empty composer that #6667 designed.

Why it happens

• resolveRouteThreadId returns null on a destination with no ?thread= → currentBranch = activeTask?.branch ?? null → picker falls back to "Select branch…".
• setCurrentTaskBranch is if (effectiveTaskId) … → no-op without a thread → selecting/New do nothing.
• VmEventsBridge's adopt-branch and auto-fresh-stale both require activeTask, so neither runs.

Relationship to #6639

Complementary, not conflicting (same file, different function — AgentInsetProvider vs VmEventsBridge; #6639 is already an ancestor). With a thread guaranteed, adopt mints a fresh unmaterialized (non-stale) branch and #6639's auto-fresh-stale check evaluates correctly instead of sitting dead on a threadless entry. No double-switch: isBranchStale(null) is false and both effects are one-shot per thread/tab.

Testing

• bun run --cwd=apps/web check ✅
• bun run lint ✅
• bun run fmt ✅
• unit: thread-route.test.ts + use-layout-state.test.ts → 57 pass / 0 fail

Not yet added: an e2e that enters a repo-backed destination without ?thread= and asserts the URL gains ?thread= + a real branch (reusing an idle empty chat when present). Happy to add if wanted.

🤖 Generated with Claude Code

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Summary by cubic

Fixes repo-backed agent editors opening without a thread in the URL, which stranded the branch picker on “Select branch…” and made selecting a branch or New silently do nothing. AgentInsetProvider now guarantees a thread before mounting by minting ?thread= into the URL, reusing the user’s idle empty chat for that agent when one exists. Scoped to repo-backed agents, the Super Agent keeps its lazy threadless composer.

^{Written for commit 50380be. Summary will update on new commits.}