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.md

@@ -36,26 +36,26 @@ We do not accept summarization of user content, cloud storage/sync features, tel
 ## Setup
 
 ```bash
-pip install -e ".[dev]"
+uv sync --extra dev   # recommended; or: pip install -e ".[dev]"
 ```
 
 ## Commands
 
 ```bash
 # Run tests
-python -m pytest tests/ -v --ignore=tests/benchmarks
+uv run pytest tests/ -v --ignore=tests/benchmarks
 
 # Run tests with coverage
-python -m pytest tests/ -v --ignore=tests/benchmarks --cov=mempalace --cov-report=term-missing
+uv run pytest tests/ -v --ignore=tests/benchmarks --cov=mempalace --cov-report=term-missing
 
 # Lint
-ruff check .
+uv run ruff check .
 
 # Format
-ruff format .
+uv run ruff format .
 
 # Format check (CI mode)
-ruff format --check .
+uv run ruff format --check .
 ```
 
 ## Project Structure