fix(hooks): MEMPAL_PYTHON override for .sh hooks' internal python3 calls

The legacy hook scripts `hooks/mempal_save_hook.sh` and
`hooks/mempal_precompact_hook.sh` shell out to `python3` for JSON
parsing and transcript-message counting. On macOS GUI launches of
Claude Code — `open -a`, Spotlight, the dock — the harness inherits
`PATH` from launchd (`/usr/bin:/bin:/usr/sbin:/sbin`), which may not
contain a `python3` at all, or may contain only a system Python that
lacks what the hook needs. The hook then fails silently in the
background log where users never look.

`mempalace` auto-ingest itself is unaffected — #340 switched that
path to the `mempalace` CLI entry point, which pipx/uv install on a
stable global PATH.

This PR adds a `MEMPAL_PYTHON` environment variable that users can
set to point the hook at any Python 3 interpreter. Resolution order
applied at each `python3` invocation site inside the two hooks:

  1. $MEMPAL_PYTHON (if set and executable)
  2. $(command -v python3) on PATH
  3. bare `python3` as a last resort

The interpreter does not need `mempalace` installed in it — only the
standard-library `json` and `sys` modules. The hook's `mempalace mine`
call runs via the CLI, independent of this override.

hooks/README.md documents the macOS GUI PATH issue and the
MEMPAL_PYTHON override. tests/test_hooks_shell.py adds 3 regression
tests (Linux/macOS only, POSIX bash):

  - MEMPAL_PYTHON override wins over PATH (proved via a
    marker-emitting shim that proxies to the real interpreter).
  - Non-executable MEMPAL_PYTHON falls back to PATH rather than
    crashing on permission denied.
  - Unset MEMPAL_PYTHON resolves via PATH.

`hooks_cli.py` (the Python implementation invoked via
`mempalace hook run ...`) already uses `sys.executable` and is
therefore trivially correct — no changes needed there.

Supersedes abandoned branch `fix/hook-bugs`.

Co-Authored-By: MSL <232237854+milla-jovovich@users.noreply.github.com>

hooks/README.md

@@ -137,6 +137,19 @@ Example output:
 
 **Hooks require session restart after install.** Claude Code loads hooks from `settings.json` at session start only. If you run `mempalace init` or manually edit hook config mid-session, the hooks won't fire until you restart Claude Code. This is a Claude Code limitation.
 
+**`MEMPAL_PYTHON` override for the hook's internal Python calls.** The save hook parses its JSON input and counts transcript messages with `python3`. When the harness is launched from a GUI on macOS — `open -a`, Spotlight, the dock — its `PATH` is the minimal `/usr/bin:/bin:/usr/sbin:/sbin` inherited from `launchd`, not your shell PATH. If `python3` isn't on that PATH, those internal calls fail and the hook can't count exchanges.
+
+Point the hook at any Python 3 interpreter to fix it:
+
+```bash
+export MEMPAL_PYTHON="/usr/bin/python3"                   # system Python is fine
+export MEMPAL_PYTHON="$HOME/.venvs/mempalace/bin/python"  # or your venv
+```
+
+Resolution priority: `$MEMPAL_PYTHON` (if set and executable) → `$(command -v python3)` → bare `python3`. The interpreter only needs `json` and `sys` from the standard library — `mempalace` itself does not need to be installed in it.
+
+Note: the `mempalace mine` auto-ingest runs via the `mempalace` CLI, so that command also needs to be on the hook's `PATH`. Installing with `pipx install mempalace` or `uv tool install mempalace` puts it on a stable global location; otherwise extend the hook environment's `PATH` to include your venv's `bin/`.
+
 ## Cost
 
 **Zero extra tokens.** The hooks notify the AI that saves happened in the background — the AI doesn't need to write anything in the chat. All filing is handled automatically. Previous versions asked the AI to write diary entries and drawer content in the chat window, which cost ~$1/session in retransmitted tokens.