smoke+docs: pin the feeder's failed-remote-mine detection; REMOTE_PATH is server-visible

First boot of the 2026-08-15 image on EMB-7KJ4VR4G shipped 7 transcripts to the
palace host and filed none. Two causes, neither visible in the log:

- MEMPALACE_PI_REMOTE_PATH was unset, so the feeder used its /data/feed default,
  which assumes a CONTAINERIZED palace server. That fleet's primary runs
  natively (systemd user unit + uv tool), so it only sees host paths and the
  mine died with "source directory not found". rsync had already succeeded.
- The feeder decided success with `'"error"' in body`, but MCP escapes the
  tool's JSON inside result.content[].text, so the check was blind and the
  catch-up log said "Done. Wing updated." Fixed in mempalace-toolkit 6e1f4f3,
  which ships `--self-test` with fixtures pinning that exact response body.

- smoke-test.sh: run `mempalace-pi-session --self-test` against the BAKED
  toolkit, so a stale or reverted MEMPALACE_TOOLKIT_REF cannot reintroduce a
  feeder that mines nothing while reporting success.
- .env.example: spell out that MEMPALACE_PI_REMOTE_PATH is the path the SERVER
  PROCESS can open — container path for a dockerized server, and identical to
  the ssh-target path for a native one — and that a mismatch fails quietly.

scripts/smoke-test.sh

@@ -195,6 +195,16 @@ run_expect "remote-palace-without-inbox skip is announced, not silent" \
     "MemPalace catch-up skipped"
 run "...and the skip notice names the variable that fixes it" \
     "grep -A6 'MemPalace catch-up skipped' /usr/local/bin/entrypoint-user.sh | grep -q 'MEMPALACE_PI_SSH_TARGET'"
+# A remote mine that FAILS must not report success. MCP answers a hard tool
+# failure with HTTP 200 and the tool's own JSON escaped inside
+# result.content[].text, so the feeder's old `'\"error\"' in body` check could
+# never see it: on 2026-08-15 a mine that died with "source directory not found:
+# '/data/feed/...'" logged "Done. Wing updated." and exited 0, and this
+# container's transcripts were filed nowhere for a whole session. The feeder
+# carries fixtures for that exact body; run them against the baked toolkit so a
+# stale/reverted toolkit ref can't reintroduce a silent feed.
+run "baked feeder detects a failed remote mine (no silent false success)" \
+    "mempalace-pi-session --self-test"
 # v1.0.0 base additions — verify presence and basic functionality.
 run "pandoc"          "pandoc --version"
 run "typst"           "typst --version"