Reorganize README, fix its command list, and check it against the binary

[zmofei]

Stacked on #62. Merge that first.

Three commits:

1. Reorganize sections. The README now follows what a reader does: install, log in, run commands. Text is moved, not rewritten.

• Uninstall moves under Install, and Auth becomes a top-level Authentication section.
• Agent skills and Generate skills are grouped under For AI agents.
• Usage is renamed Global options. Update notices, history and logs get their own section.
• Privacy becomes top-level. every_telemetry_marker_is_disclosed now finds it by ## Privacy. I checked that it still fails when a disclosure is removed and when the section is missing.
• Every heading other docs link to keeps its anchor.

2. Check README's commands against the binary. Two tests in tests/docs_contract.rs:

• Every mapbox … in a shell block or inline code span must be a real command from mapbox --schema.
• Every top-level command appears at least once.

Run against the README before commit 3, they report the four fake groups and the three missing commands below. They also catch a typo in a nested subcommand (styles draft gett).

3. Fix the command list and trim.

• The API group list named rasterarrays, static-images, static-tiles and tilequery, which the binary doesn't have, and left out static. doctor, usage and config weren't mentioned; a new Diagnostics and settings section covers them.
• 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, and the installer-test note moves to CONTRIBUTING.md.
• Update notices, history, logs and Privacy are left as they were. docs/commands.md links to the README for the first three, and Privacy is legal text.

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

Unrelated, 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 such as mapbox usage --help. Left alone in case it's intentional.