Make Tau updates prominent and add tau update

[alejandro-ao]

Summary

• render available-version notices as the first TUI transcript item in bright yellow
• add tau update, which detects and reuses the installer that owns Tau's active environment
• make uv-tool updates install the explicit latest stable version so an old receipt pin cannot block upgrades
• point startup guidance and installation docs to the new command

User experience

Outdated Tau installations are now difficult to miss, without interrupting startup. Users can upgrade directly with one memorable command. Tau respects the existing installation: uv tool, uv pip, pipx, and pip installs are updated with that same manager, while direct-source, Conda/Pixi, and unknown installs stop with safe instructions instead of trying unrelated tools.

Issue

No linked issue.

Manual validation

1. Seed ~/.tau/cache/update-check.json with a fresh latest_version newer than the installed version.
2. Run tau and confirm the bright-yellow update notice is the first transcript item, before release notes and session history.
3. Run tau update from a normal uv, pipx, or pip installation and confirm Tau invokes that installation's manager.
4. For a uv tool originally installed with an exact old version, confirm Tau runs uv tool install tau-ai@<latest-version> rather than preserving the old pin.
5. Confirm an editable checkout reports its original source instead of modifying another installation.
6. Run tau --version after updating.

Checks

• uv run pytest
• uv run ruff check .
• uv run ruff format --check .
• uv run mypy
• uv build --out-dir /tmp/tau-feature-dist
• hugo --minify --destination /tmp/tau-feature-site (from website/)