fix: pre-commit is a development tool, not a runtime dependency (#250)

`pre-commit` was added to `[project].dependencies` in #98, alongside the
black hook. Nothing in spatialmath imports it, but every `pip install
spatialmath-python` has installed it, and its dependency tree
(virtualenv, identify, cfgv, nodeenv, ...), ever since. The hook config
was later removed (#153) and restored, but the runtime dependency
stayed.

**Why it matters beyond bloat:** it makes the install fail wherever a
transitive pin conflicts. `virtualenv` 21.14.7 now requires
`packaging>=26.3`, and Pyodide ships `packaging` 24.2, so in JupyterLite
/ in-browser notebooks `micropip.install("spatialmath-python")` raises
`ValueError: Requested 'packaging>=26.3', but packaging==24.2 is already
installed`. That breaks the in-browser demos of the Robotics, Machine
Vision and bdsim toolboxes, and it will break again whenever any dev
tool in that tree raises a floor.

**Change:** move `pre-commit` to the `dev` extra (CI already installs
`.[dev]`) and make the README's contributor instructions install it.

**Verified:** built the wheel and installed it with `micropip.install`
in Pyodide 0.27.6. Dependencies resolve, and `import spatialmath` and
`SE3.Rx(...)` work. Before the change the same install fails with the
error above.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-authored-by: Claude Sonnet 5.5 <noreply@anthropic.com>

Author: petercorke

README.md

@@ -137,6 +137,8 @@ git clone https://github.com/rai-opensource/spatialmath-python.git
 cd spatialmath-python
 pip install -e .
 # Optional: if you would like to contribute and commit code changes to the repository,
+# install the development tools (including pre-commit) and set up the git hooks
+# pip install -e .[dev]
 # pre-commit install
 ```
 

pyproject.toml

@@ -40,7 +40,6 @@ dependencies = [
     "matplotlib",
     "ansitable",
     "typing_extensions>=4.0.0",
-    "pre-commit",
 ]
 
 [project.urls]
@@ -52,6 +51,7 @@ dependencies = [
 [project.optional-dependencies]
 
 dev = [
+    "pre-commit",
     "sympy",
     "pytest",
     "pytest-timeout",