feat: add per-command help

[Bccorb]

seamless <command> -h / --help (and seamless help <command>) now print help for that command instead of the full manual.

$ seamless verify -h

seamless verify — seamless v0.10.2

USAGE

  seamless verify [--local] [--api-only] [--no-react] [--filter=<flow>] [--keep-up]

DESCRIPTION

  Stand up the auth stack and run the conformance suite across the API and
  the cookie (adapter) paths. Requires Docker. ...

  --api-only
    • Run the API layer only, skipping the adapter and browser layers
  ...

What changed

• One registry, two renderings. src/commands/helpTopics.ts holds an entry per command (usage lines, description sections, examples). Both the full seamless --help output and the per-command output render from it, so a flag is documented once. COMMANDS is derived from the same registry, replacing the hardcoded list the dispatcher used for its unknown-command message.
• src/commands/help.ts now formats instead of hardcoding: printHelp() composes the USAGE and COMMANDS sections, printCommandHelp(name) prints the scoped view and returns false for an unknown topic so the caller falls back to the unknown-command error.
• The help check runs before a command parses its own args (src/index.ts). That also fixes a bug: seamless init -h previously fell through init's positional parsing and was treated as a project name, so it would have scaffolded ./-h.
• hasHelpFlag (src/core/args.ts) stops at a literal --, leaving room for a command that legitimately takes -h as a value: seamless config set key -- -h.

Top-level seamless --help output is unchanged apart from a new seamless <command> --help usage line, and --no-react on verify, which was a real flag that had never been documented.

Checks

• npm run build clean.
• npm test: 718 passed, 4 skipped.
• npm run coverage passes; 100% on index.ts, help.ts, helpTopics.ts, and args.ts.

New tests cover the flag parsing (including the -- escape), every command's scoped output, and the dispatcher paths (init -h not scaffolding, help <command>, unknown topic).

README gained a "Getting help" section, AGENTS.md points future changes at the registry, and there is a changeset (minor).