docs(install): recommend uv as the package manager

End-user installs now lead with `uv tool install mempalace`, with
`pip install mempalace` kept as a fallback. Dev/contributor docs lead
with `uv sync --extra dev` and `uv run` for tests/benchmarks/lint, with
the equivalent pip recipe kept inline. The shipped `/mempalace:init`
skill instructions (mempalace/instructions/init.md) try `uv tool install`
first when uv is on PATH, then fall back through the pip variants.

Adds a .python-version pin at 3.12 because the lockfile's
onnxruntime==1.24.3 only ships wheels for Python >=3.11; without the
pin, `uv sync` on a host where uv prefers 3.10 fails with no source
distribution available, which would make the documented command a
footgun. pyproject's `requires-python = ">=3.9"` is unchanged — pip
users on 3.9/3.10 are unaffected.

Files updated: README.md, CONTRIBUTING.md, CLAUDE.md, the gemini-cli
guide and example, the .claude-plugin / .codex-plugin READMEs, the
mempalace SKILL, the openclaw SKILL, tools/save.md, the three
benchmarks docs, and the corresponding website mirrors.

website/guide/gemini-cli.md

@@ -9,22 +9,30 @@ MemPalace works natively with [Gemini CLI](https://github.com/google/gemini-cli)
 
 ## Installation
 
+We recommend [`uv`](https://docs.astral.sh/uv/) — it creates and manages the
+virtual environment for you:
+
 ```bash
 # Clone the repository
 git clone https://github.com/MemPalace/mempalace.git
 cd mempalace
 
-# Create a virtual environment
-python3 -m venv .venv
+# Create the venv and install MemPalace + dependencies
+uv sync
+```
 
-# Install dependencies
+This produces a `.venv/` directory with the project installed in editable
+mode. If you prefer plain pip, the equivalent is:
+
+```bash
+python3 -m venv .venv
 .venv/bin/pip install -e .
 ```
 
 ## Initialize the Palace
 
 ```bash
-.venv/bin/python3 -m mempalace init .
+uv run python -m mempalace init .
 ```
 
 ### Identity and Project Configuration (Optional)
@@ -88,7 +96,7 @@ Once connected, Gemini CLI will automatically:
 
 Mine existing code or docs:
 ```bash
-.venv/bin/python3 -m mempalace mine /path/to/your/project
+uv run python -m mempalace mine /path/to/your/project
 ```
 
 ### Verification