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.

.codex-plugin/README.md

@@ -6,7 +6,7 @@ Give your AI a persistent memory -- mine projects and conversations into a searc
 
 - Python 3.9+
 - Codex CLI installed and configured
-- `pip install mempalace`
+- `uv tool install mempalace` (recommended) or `pip install mempalace`
 
 ## Installation
 
@@ -39,10 +39,10 @@ git clone https://github.com/MemPalace/mempalace.git
 cd mempalace
 ```
 
-2. Install the Python package:
+2. Install the Python package (uv recommended):
 
 ```bash
-pip install -e .
+uv sync   # or: pip install -e .
 ```
 
 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.