feat(cli): add --help flag with rich usage output

[EdwardIrby]

Summary

Adds a --help / -h flag to the ydc CLI with a sectioned usage block, following the plaited CLI pattern.

Replaces the minimal 3-line usage string with a structured help block (Commands, Tools, Flags, Environment) and adds -h as a short alias for --help.

Behavior change

• ydc --help / ydc -h → help to stdout, exit 0 (deliberate success)
• ydc (no command) → help to stderr, exit 1 (nothing actionable given)
• ydc <unknown> → unchanged: error to stderr, exit 1

This keeps stdout reserved for JSON results or explicit help, so script/agent callers can reliably distinguish a successful command from a missing one.

Test plan

• prints plain-text help for --help
• prints the same help for the -h short flag
• lists every allowlisted tool and flag in the help output
• prints help to stderr and exits non-zero when no command is given
• rejects unknown commands immediately
• Existing 14 tests unchanged and passing
• check:types and check:biome clean

References

• Example pattern: https://github.com/plaited/plaited/blob/dev/src/cli/cli.ts

[factory-droid]

Droid finished @EdwardIrby's task —— View job

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Droid is reviewing code and running a security check…

Review complete. Clean PR that adds a structured --help/-h flag to the ydc CLI and correctly separates the no-command case (stderr, exit 1) from the explicit help request (stdout, exit 0). Control flow, test coverage, and documentation are all consistent with the stated behavior. No issues found.