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.

.claude-plugin/README.md

@@ -23,7 +23,7 @@ claude plugin add /path/to/mempalace
 
 ## Post-Install Setup
 
-After installing the plugin, run the init command to complete setup (pip install, MCP configuration, etc.):
+After installing the plugin, run the init command to complete setup (installs the `mempalace` package via `uv tool` or `pip`, configures MCP, etc.):
 
 ```
 /mempalace:init

.claude-plugin/skills/mempalace/SKILL.md

@@ -16,10 +16,10 @@ Ensure `mempalace` is installed:
 mempalace --version
 ```
 
-If not installed:
+If not installed (uv recommended):
 
 ```bash
-pip install mempalace
+uv tool install mempalace   # or: pip install mempalace
 ```
 
 ## Usage