port: fix the credential mount, the /reborn pull, and cargo re-fetching

Three fixes from the port agent's first infrastructure report.

**A. The credential file is written, so it cannot be a read-only mount.**
`credential.helper=store` rewrites its file after a successful auth: temp file,
then rename over the target. Renaming onto a bind-mount point gives EBUSY, which
surfaces as `fatal: unable to write credential store: Device or resource busy`.

The push succeeds anyway, and that is the real hazard -- a `fatal:` line that is
routinely wrong teaches the reader to ignore the one that is real. It also fired
intermittently, so it read as flakiness rather than as a mount.

Fixed by mirroring the pattern already used for .claude.json: mount it as
`.git-credentials.host:ro` and have the entrypoint copy it to a writable
~/.git-credentials at 600. Mounting rw would also silence it, but then the
container can clobber the host's real credential file; copying cannot.

**B. `git -C /reborn pull` can never work, and should not.** /reborn is a live
read-only mount of the RE agent's working tree -- it updates itself, and pulling
would move another agent's checkout. The prompt now says so, and adds the
consequence the agent found the hard way: because the mount is live, HANDOFF can
move mid-iteration, so anything copied out of it (BLOCKED.md especially) may
already be stale and must be re-checked rather than trusted.

**C.** CARGO_HOME moves to a named volume; it was on the container overlay, so
the pinned decoder source was re-fetched from the network on every fresh start.

Also adds SYLPH_PORT_REPO, so this launcher can be run from a worktree without
repointing the agent's checkout -- which is how these edits were made, the agent
being mid-iteration on auto/p0-exporter in the shared tree.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

docs/loop-task.md

@@ -16,8 +16,16 @@ believed later. If you need an answer the disc has not given you, write it in
 
 1. `docs/MISSION.md` — milestones, gates, scope.
 2. `/reborn/docs/port/HANDOFF.md` — **the contract.** What is decoded, what was
-   measured off the running game, and what is known undecodable. `git -C /reborn
-   pull` first; the RE agent publishes continuously.
+   measured off the running game, and what is known undecodable.
+
+   **It is a live read-only mount of the RE agent's working tree**, so it updates
+   itself and there is nothing to pull — `git -C /reborn pull` cannot work (the
+   mount is read-only) and should not: it would move another agent's checkout.
+   `git -C /reborn log -1` shows where they are.
+
+   Because it is live, **it can move under you mid-iteration.** Anything you
+   copied out of it earlier — `docs/BLOCKED.md` especially — may already be
+   stale. Re-check it against HANDOFF before trusting it.
 3. `docs/FORMAT.md` — the open format. It is versioned and it is yours to
    revise, but a change is a deliberate act with a version bump.
 4. `docs/BLOCKED.md` — what you are waiting on, so you do not re-discover it.