fix: address Copilot review on PATH visibility (PR #1414)

- mempalace/instructions/init.md: only skip Step 4 when `mempalace
  --version` succeeds. `pip show` / `uv tool list` reporting an install
  is not enough -- if the package lives in an unactivated venv, Step 5
  (`mempalace init ...`) fails with command-not-found. Treat that case
  as not-installed and re-install via Step 3 into a PATH-visible
  location.
- .codex-plugin/README.md: switch the git-install recipe from `uv sync`
  to `uv tool install --editable .` so the bundled `plugin.json`
  (which invokes `mempalace-mcp` by bare name) can launch the MCP
  server. Plain `uv sync` only puts the script in `.venv/bin/`, which
  Codex won't find unless the venv is activated first.

.codex-plugin/README.md

@@ -39,12 +39,17 @@ git clone https://github.com/MemPalace/mempalace.git
 cd mempalace
 ```
 
-2. Install the Python package (uv recommended):
+2. Install the Python package so the `mempalace-mcp` script lands on
+   your PATH (the bundled `plugin.json` invokes it by bare name):
 
 ```bash
-uv sync   # or: pip install -e .
+uv tool install --editable .   # or: pip install -e .
 ```
 
+   Plain `uv sync` is **not** enough here — it installs the scripts into
+   `.venv/bin/`, which Codex will not find unless you activate the venv
+   before launching Codex.
+
 3. The `.codex-plugin` directory is already in the repo root. Codex CLI will detect it automatically when you run Codex from inside the repository.
 
 4. Initialize your palace:

mempalace/instructions/init.md

@@ -11,12 +11,15 @@ tell the user they need Python 3.9+ installed and stop.
 
 ## Step 2: Check if mempalace is already installed
 
-Run `mempalace --version` to see if the CLI is already on the user's PATH.
-If it succeeds, report the installed version and skip to Step 4.
+Run `mempalace --version`. If it succeeds, the CLI is on PATH — report
+the installed version and skip to Step 4.
 
-If that fails, fall back to `pip show mempalace` (and `uv tool list` if `uv`
-is available) to detect an existing install. If found, report it and skip
-to Step 4.
+If `mempalace --version` fails, **do not** skip to Step 4 just because
+`pip show mempalace` or `uv tool list` reports the package as installed:
+the package may live inside a venv that isn't activated, in which case
+Step 5 (`mempalace init ...`) will fail with `command not found`. Treat
+that case as not-installed and continue to Step 3, which will (re)install
+into a PATH-visible location via `uv tool install` or `pip`.
 
 ## Step 3: Install mempalace