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.

examples/gemini_cli_setup.md

@@ -9,17 +9,24 @@ This guide explains how to set up MemPalace as a permanent memory for the [Gemin
 
 ## 1. Installation
 
-On many Linux systems, installing Python packages globally is restricted. We recommend using a local virtual environment within the MemPalace directory.
+On many Linux systems, installing Python packages globally is restricted. We
+recommend [`uv`](https://docs.astral.sh/uv/), which creates and manages a
+local virtual environment for you.
 
 ```bash
 # Clone the repository (if you haven't already)
 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 in editable mode
+uv sync
+```
 
-# Install dependencies and MemPalace in editable mode
+This produces a `.venv/` directory inside the repo with everything installed.
+If you prefer plain pip:
+
+```bash
+python3 -m venv .venv
 .venv/bin/pip install -e .
 ```
 
@@ -29,7 +36,7 @@ Set up your "Palace" (the database) and configure your identity.
 
 ```bash
 # Initialize the palace in the current directory
-.venv/bin/python3 -m mempalace init .
+uv run python -m mempalace init .
 ```
 
 ### Identity and Wings (Optional but Recommended)
@@ -86,7 +93,7 @@ Once connected, Gemini CLI will automatically:
 ### Manual Mining
 If you want the AI to learn from your existing code or docs immediately, run the "mine" command:
 ```bash
-.venv/bin/python3 -m mempalace mine /path/to/your/project
+uv run python -m mempalace mine /path/to/your/project
 ```
 
 ### Verification