S12: the command reference as a published docs artifact, every example run #16

Part of #17. Design: docs/design.md §7.4.

Scope

The command's reference, published as a docs artifact with each release so the Relix site can render a Command line section from a pinned version:

• One page per command, generated from the same picocli CommandSpec as --help and the man pages, so the three cannot drift.
• Hand-written pages:
  • catalog discovery, overlay and trust
  • input binding
  • output formats
  • secrets and profiles
  • pipelines with jq, find and xargs
• A machine-readable index of the pages.

Every example in those pages runs in a test here, over a fixture .relix/ tree and fixture files, comparing stdout and the exit status. The examples guard is probed by mutation when it is written.

User-facing rule: the pages state what the command does. A limitation is stated as a fact, with no promise of future work.

Dependencies

• S1–S9 for the command surface.
• The pages for -i and -a wait on S3 (S3: bind data inputs with -i NAME=[FORMAT:]LOCATION, stdin included #7) and S10 (S10: bound query parameters with -a NAME=VALUE #14).

Acceptance

• Every command has a page.
• No internal link is broken.
• Every example passes.