fix(hooks): consolidate transcript ingest, harden shell parsers (#1231 review)

Address Copilot review on #1231:

1. Stop double-mining the transcript on the Python side. ``_get_mine_targets``
   now returns only the ``MEMPAL_DIR`` projects target — the convos target
   for the transcript dir is dropped because ``_ingest_transcript`` already
   handles it on every hook fire. The duplicate spawn was using
   ``sys.executable`` (vs ``_mempalace_python()``) and a different ``--wing``,
   so each Stop/PreCompact event was writing the same transcript into two
   wings under asymmetric interpreters and overwriting the single
   ``_MINE_PID_FILE`` lock.

2. ``_maybe_auto_ingest`` and ``_mine_sync`` now spawn via
   ``_mempalace_python()`` so the resolved interpreter matches the venv
   that owns mempalace (matters under GUI-launched harnesses where
   ``sys.executable`` may resolve to a system Python without chromadb).

3. Replace ``eval $(...)`` in both shell hooks with a ``mapfile``-based
   reader. Sanitized values are still emitted by the same Python parser,
   but the shell now does plain variable assignment instead of executing
   the parser's stdout — smaller blast radius if the sanitizer is ever
   bypassed.

4. Mirror ``_validate_transcript_path`` in the shell hooks via a
   ``is_valid_transcript_path`` helper — extension + traversal-segment
   rejection, parity with the Python validator. The convos mine in each
   shell hook is now gated on the validator instead of bare ``-f``.

5. Tighten the ``..`` traversal test that previously exercised the
   suffix gate by mistake (``../../etc/passwd`` lacks ``.json[l]``).
   Use ``.jsonl`` paths with traversal segments to actually hit the
   ``..`` rejection branch.

6. README: add a one-liner pointing at ``mempalace sweep`` for users
   who want per-message recall on top of the file-level chunks the
   hooks produce. The sweeper was undiscoverable previously.

Tests: 1418 passed, 1 skipped (full suite minus benchmarks).

README.md

@@ -149,6 +149,10 @@ system prompt:
 Two Claude Code hooks save periodically and before context compression:
 [mempalaceofficial.com/guide/hooks](https://mempalaceofficial.com/guide/hooks.html).
 
+For per-message recall on top of the file-level chunks the hooks produce,
+run `mempalace sweep <transcript-dir>` periodically — it stores one
+verbatim drawer per user/assistant message, idempotent and resume-safe.
+
 ---
 
 ## Requirements