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>

docker/entrypoint.sh

@@ -32,6 +32,14 @@ fi
 if [ -f "$HOME/.claude.host.json" ] && [ ! -s "$HOME/.claude.json" ]; then
   cp "$HOME/.claude.host.json" "$HOME/.claude.json" 2>/dev/null || true
 fi
+# Same reason as .claude.json above: `credential.helper=store` rewrites this
+# file by rename-over-target, which fails with EBUSY on a bind mount. Copy it to
+# a writable path; nothing is ever written back to the host's file.
+if [ -f "$HOME/.git-credentials.host" ]; then
+  cp "$HOME/.git-credentials.host" "$HOME/.git-credentials" 2>/dev/null || true
+  chmod 600 "$HOME/.git-credentials" 2>/dev/null || true
+fi
+
 CLAUDE_VER=$(claude --version 2>/dev/null | grep -oE '^[0-9][0-9.]*' || echo 0.0.0)
 python3 /usr/local/bin/seed-claude-config.py "$HOME/.claude.json" "$CLAUDE_VER" \
         "$PWD" "${PROJECT_DIR:-/work}" "$HOME" || true