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.

README.md

@@ -49,11 +49,16 @@ Architecture, concepts, and mining flows:
 
 ## Install
 
+We recommend [`uv`](https://docs.astral.sh/uv/) — `uv tool install` puts
+the `mempalace` CLI in an isolated environment on your PATH:
+
 ```bash
-pip install mempalace
+uv tool install mempalace
 mempalace init ~/projects/myapp
 ```
 
+If you prefer pip, `pip install mempalace` still works.
+
 ## Quickstart
 
 ```bash
@@ -120,9 +125,9 @@ own research page for their published numbers.
 ```bash
 git clone https://github.com/MemPalace/mempalace.git
 cd mempalace
-pip install -e ".[dev]"
+uv sync --extra dev   # or: pip install -e ".[dev]"
 # see benchmarks/README.md for dataset download commands
-python benchmarks/longmemeval_bench.py /path/to/longmemeval_s_cleaned.json
+uv run python benchmarks/longmemeval_bench.py /path/to/longmemeval_s_cleaned.json
 ```
 
 ---