Fix README's command list and trim duplicated detail

[zmofei]

Stacked on #63. Merge #62 and #63 first.

Fixes a wrong command list. README's API group list named rasterarrays, static-images, static-tiles and tilequery, none of which the binary has, and left out static. It also never mentioned doctor, usage or config. The list is corrected, and a new Diagnostics and settings section shows those three plus history.

Adds a check so it can't drift again. Two tests in tests/docs_contract.rs:

• every_command_the_readme_spells_exists: every mapbox … line in a shell block or inline code span must name a real command from mapbox --schema.
• every_top_level_command_appears_in_the_readme: every top-level command appears at least once.

Run against #63's README, they report exactly the four fake groups and the three missing commands. They also catch a typo in a nested subcommand (styles draft gett). The check is in its own commit, so reverting the README commit shows it failing.

Trims duplicated detail. Agent skills, completion, generate-skills and Tileset CLI are cut to examples plus a link, since docs/commands.md covers each fully. Implementation notes are gone (e.g. why agent-skills has no lock file), and the PowerShell aside is shorter. The installer-test note moves to CONTRIBUTING.md. Update notices, history, logs and Privacy stay as they were: docs/commands.md links to the README for the first three, and Privacy is legal text.

Not checked: the new tests only cover command paths, not flags or descriptions in README prose.

Unrelated but found along the way: REPO_URL in src/main.rs and repository in Cargo.toml point at github.com/mapbox/cli, which redirects to mapbox/cli-opensource-preview, not this repo. It shows up in help text (for example mapbox usage --help). I left it alone in case that's intentional.

[zmofei]

Folded into #63. This shows as merged only because its commits landed on #63's branch; nothing reached main from here.