fix(loop): worktree provisioning, fresh fetch, corruption halt

Three P1 adoption-wave findings, all of which break an unattended run.
The worktree needs make build-frontend and make testdata once before its
first push — the pre-push go-test hook embeds frontend/dist and the
fixture library and refused the push without them. The merge leg now
says to fetch origin/main in the same breath as the refresh merge: a
cached origin/main merged against the wrong base, CI went green on it,
and the merge came back 405 "behind base" — a full cycle wasted. And a
killed fetch leaves 0-byte objects in the shared store whose signature
(error: object file … is empty / unpack-objects failed) must halt the
loop for a human instead of churning, with the repair recipe written
down.

Closes #240

.planning/plans/active/020-autonomous-backlog-loop.md

@@ -112,7 +112,9 @@ in the job's directory — that limitation is the switch:
 
 - **Worktree:** `git worktree add` a dedicated clone at
   `~/.paseo/worktrees/loop/jumpy-hound`. Loop edits happen only there; a
-  dirty tree there is the loop's business and nobody else's.
+  dirty tree there is the loop's business and nobody else's. **Provision
+  it once before its first push:** `make build-frontend` + `make testdata`
+  — the pre-push `go-test` hook needs both and refuses without them.
 - **Session:** pi in that worktree, `/name loop`. The job is bound to that
   session, so another pi elsewhere in the same directory does not
   double-fire it.