feat(help): add hidden --help-json for machine-readable command tree

[crowlbot]

--help is written for humans; an agent that wants to know which subcommands
exist, which flags each one takes, and which of those are required has to scrape
it. This adds a hidden --help-json global flag that dumps the command tree as
one JSON object on stdout and exits 0.

• help-json.ts — serializes a Cliffy command: description, positional
  arguments, options (flags, required, collect, global, hidden, static
  default), examples, and subcommands recursively. Hidden commands/options are
  included but flagged, so the dump is the full surface.
• Handled in main.ts before parse(), so introspecting a command that
  declares required options (setup-aws --help-json) doesn't trip validation.
• Leading positional args address a subtree: deno deploy env list --help-json
  returns just that node. Inherited globals are materialized on every node, so a
  single node lists every flag it accepts.
• Works on the standalone deno sandbox root too.
• Declared as a hidden globalOption on both roots purely so it shows up in the
  dumped tree; the pre-parse interception is what implements it.

Tests in tests/cli_contract.test.ts (token-free): tree dump, global agent
flags present and marked global, every command/option has a description,
subtree resolution, required-option command introspects cleanly, sandbox root.

Also documents it in AGENTS.md.

[crowlbot]

CI here is red on three tests that are not caused by this PR:

• the two tests/cli_contract.test.ts unknown-flag failures are pre-existing on main (cliffy 1.2 swallows unknown options into positional slots),
• --help-json: every command and option has a description catches tunnel-login missing a .description().

Both are fixed in #136. Once that lands I will rebase this branch and CI should go green.