Refactor for UV!

[EVWorth]

## Summary

Replaces ad-hoc Python environment bootstrapping in CI with a proper `uv`-managed project, removing manual setup steps and aligning the repo with standard `uv` conventions.

## Changes

### `pyproject.toml` (new)
Declares the project and its dependency groups. The `docs` group pins `zensical`, which was previously installed imperatively via `pip install uv && uv pip install --system zensical`. The `requires-python = "~=3.12.0"` constraint replaces the per-command `--python 3.12` flags scattered across workflows and tool docstrings.

### CI workflows
Both `build-docs.yml` and `sync-docs-discourse.yml`:
- Replace the two-step `actions/setup-python` + `pip install uv` setup with `astral-sh/setup-uv@v7`
- `build-docs` now runs `uv run --group docs zensical build`, resolving deps from `pyproject.toml`
- Drop `--python 3.12` from all `uv run` invocations (covered by `pyproject.toml`)
- Fix `workflow_dispatch` input defaults from string `"false"` to boolean `false`

### `.gitignore`
Adds `uv.lock` to prevent an accidental unannounced lockfile commit.

### `tools/` docstrings
Updates `Run:` instructions in all five test files and `sync_to_discourse.py` from `python3 tools/...` to `uv run python tools/...`.

## Why

The previous approach bootstrapped `uv` inside each workflow job manually, then bypassed the project model entirely with `--system`. This makes the repo a proper `uv` project so dependency management is declarative, reproducible, and consistent between local dev and CI.

## Testing

- `uv run --group docs zensical build` — verified locally ✓
- All CI test steps use `uv run python tools/test_*.py` with no version flag, relying on `pyproject.toml`'s `requires-python` constraint

[Copilot]

Pull request overview

Refactors the repo’s Python usage in CI and tooling to follow a uv-managed project model, moving dependency/spec configuration into pyproject.toml and updating workflows and tool instructions accordingly.

Changes:

• Adds a pyproject.toml defining Python requirements and a docs dependency group for zensical.
• Updates CI workflows to use astral-sh/setup-uv@v7 and run commands via uv run (including --group docs for doc builds).
• Updates tool/test docstrings to recommend running via uv run, and ignores uv.lock.

Reviewed changes

Copilot reviewed 9 out of 10 changed files in this pull request and generated 3 comments.

Show a summary per file

File	Description
pyproject.toml	Introduces uv project metadata, Python requirement, and docs dependency group.
.github/workflows/build-docs.yml	Switches doc build job setup to setup-uv and runs build via uv run --group docs.
.github/workflows/sync-docs-discourse.yml	Switches test/sync jobs to setup-uv, updates workflow_dispatch boolean defaults, and runs tools via uv run.
.gitignore	Adds uv.lock to ignored files.
tools/sync_to_discourse.py	Updates usage instructions to remove explicit --python 3.12 flags.
tools/test_converter.py	Updates run instructions to uv run python ....
tools/test_discourse_client.py	Updates run instructions to uv run python ....
tools/test_nav_parser.py	Updates run instructions to uv run python ....
tools/test_content_cache.py	Updates run instructions to uv run python ....
tools/test_sync_to_discourse.py	Updates run instructions to uv run python ....

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