docs: correct macOS troubleshooting Python floor to 3.10+ (#981)

## Description

`wiki/macos-deployment.md` told users that Headroom "Requires Python
3.9+", but the project's actual floor is Python 3.10. This corrects that
one line to 3.10+ so it matches `pyproject.toml` and every other doc.

Closes #

## Type of Change

- [x] Bug fix (non-breaking change that fixes an issue)
- [ ] New feature (non-breaking change that adds functionality)
- [ ] Breaking change (fix or feature that would cause existing
functionality to change)
- [x] Documentation update
- [ ] Performance improvement
- [ ] Code refactoring (no functional changes)

## Changes Made

- `wiki/macos-deployment.md` (troubleshooting → "Common causes"):
`Requires Python 3.9+` → `Requires Python 3.10+`.

## Testing

<!-- Check what you actually ran, then paste the real command output
below. -->

- [ ] Unit tests pass (`pytest`)
- [ ] Linting passes (`ruff check .`)
- [ ] Type checking passes (`mypy headroom`)
- [ ] New tests added for new functionality
- [x] Manual testing performed

### Test Output

```text
# Ground truth — the real Python floor:
$ grep -n 'requires-python' pyproject.toml
11:requires-python = ">=3.10"

# Every other doc already says 3.10+, and this was the only "3.9" left:
$ grep -rniE 'requires? python *3\.(9|10)' README.md docs/ wiki/ CONTRIBUTING.md
README.md: ... Requires **Python 3.10+**.
docs/content/docs/installation.mdx:16: Headroom requires **Python 3.10+** ...
docs/content/docs/installation.mdx:245: This project requires **Python 3.10+**.
wiki/index.md:405: Requires Python 3.10+.
CONTRIBUTING.md:125: - Python 3.10+. ...
wiki/macos-deployment.md:426: - Python version incompatible: Requires Python 3.10+   # fixed by this PR
```

## Real Behavior Proof

- Environment: local clone at `origin/main`; this is a
documentation-only change.
- Exact command / steps: confirmed `pyproject.toml` declares
`requires-python = ">=3.10"`; grepped all docs and found
`wiki/macos-deployment.md` was the only file claiming `3.9+`; changed
that single line to `3.10+`.
- Observed result: all Python-floor mentions across README,
`docs/content/docs/installation.mdx`, `wiki/index.md`,
`CONTRIBUTING.md`, and now `wiki/macos-deployment.md` agree on 3.10+,
matching `requires-python`.
- Not tested: N/A — single-line prose fix in a Markdown file; no code
paths, no build, no runtime behavior involved.

## Review Readiness

- [x] I have performed a self-review
- [x] This PR is ready for human review

## Checklist

- [x] My code follows the project's style guidelines
- [x] I have performed a self-review of my code
- [ ] I have commented my code, particularly in hard-to-understand areas
- [x] I have made corresponding changes to the documentation
- [x] My changes generate no new warnings
- [ ] I have added tests that prove my fix is effective or that my
feature works
- [x] New and existing unit tests pass locally with my changes
- [ ] I have updated the CHANGELOG.md if applicable

## Additional Notes

Documentation-only change, so `pytest`/`ruff`/`mypy` over the `headroom`
package are N/A — the diff contains no Python source. The fix was a
misleading minimum specifically in the version-incompatibility
troubleshooting step, where the wrong floor (3.9 vs the real 3.10) would
actively mislead a user diagnosing a Python version problem on macOS.

wiki/macos-deployment.md

@@ -423,7 +423,7 @@ tail -f ~/Library/Logs/headroom/proxy-error.log
 
 - Missing dependencies: `pip install headroom-ai[proxy]`
 - Invalid API key: Verify `ANTHROPIC_API_KEY`
-- Python version incompatible: Requires Python 3.9+
+- Python version incompatible: Requires Python 3.10+
 
 ### ANTHROPIC_BASE_URL Not Set