fix(copilot): show over-limit AI credits

[vmvarela]

Summary

• Show Copilot Business token-billing usage after the included credit limit is exceeded, instead of falling back to a plan-only row.
• Use GitHub-reported credits_used as AI Credits when valid; retain a guarded derived Premium Interactions fallback when that field is missing or invalid.
• Add parser/provider regressions and clarify the normal versus over-limit behavior in the provider docs.

Linked Issue

Fixes #300

OpenCode Validation

• Current production released OpenCode version tested: v2.0.20
• Why this version is relevant to the fix: the server loaded the local package build and queried the active Copilot OAuth connection.

Before-and-after evidence

Before (/quota_status):

Plan: business | quota details unavailable

After (live local build):

Credits: 3,739/3,000 percent_remaining=0 reset_at=2026-10-01T00:00:00.000Z

Before/after TUI screenshots: comment #5908364360.

Surface checks:

• TUI sidebar: before-and-after screenshots attached in the linked comment.
• /quota_status: live probe verified against the local package.
• Web/command output: not separately tested in this follow-up.
• Toast: not separately tested.
• Compact line below the message input: not separately tested.

Quality Checklist

• I ran pnpm verify
• This change is focused and avoids unrelated behavior changes
• I updated or added tests when behavior changed
• I updated docs when user-facing workflow, command, or config behavior changed
• For provider changes, I followed Provider Changes; this is a Copilot parser/rendering fix, not a new provider.

[vmvarela]

Before:

After: