Add usage transparency plugin for quota troubleshooting

[teee32]

Problem

• Users often conflate temporary rate limits, quota exhaustion, auth conflicts, and organization/subscription access failures.
• That makes Claude Code failures harder to diagnose, especially when the CLI only exposes local signals and not backend billing state.

Change

• Add an example plugin, usage-transparency, with /usage-status and /usage-help.
• These commands focus on classifying the failure mode, separating observed local facts from unknown platform-side state, and giving targeted next steps.

Scope boundary / non-goals

• This PR adds repo-scoped guidance only.
• It does not add backend billing or quota visibility.
• It does not change core Claude Code CLI or TUI runtime behavior.

Test plan

• Manually reviewed the plugin README and command markdown for accuracy and consistency.
• Checked representative examples to ensure rate limit stays distinct from quota exhausted.
• Verified the docs explicitly state when quota, reset time, or entitlement details remain unknown locally.
• Confirmed the change is limited to plugin/docs packaging, with no core runtime behavior changes.

[teee32]

Updated the PR to make the scope more explicit:
1.positioned this as a local-signal troubleshooting layer, not a replacement for native usage/billing visibility
2.removed assumptions around /extra-usage
3.tightened wording to avoid implying backend entitlement visibility
4.added discoverability in the root/plugin READMEs