docs(devtools/claude-usage): README, devtools index, and packaging polish

[jaeyeom]

Parent

Part of #199 — port claude-usage into devtools/.

Do this once MVP CLI + credentials + API client are usable end-to-end.

Scope

• Write devtools/claude-usage/README.org covering:
  • purpose (5-hour / 7-day Claude Code usage windows)
  • install (go install ...)
  • usage examples (human + --json)
  • prerequisites (Claude Code OAuth login on macOS; not for pure API-key users)
  • security notes (read-only credential use; token never printed)
  • exit codes
• Add an entry under Development Utilities in devtools/README.org (tag e.g. experimental or active)
• Ensure make check / tests pass for the new package
• Optional: note that ~/.local/bin/claude-usage can be replaced by the installed binary

Acceptance criteria

• README.org is complete enough for a new user to install and run
• devtools/README.org indexes the tool
• CI-friendly tests green under the repo’s usual check path
• No remaining hard dependency on claude-hud plugin cache for the happy path