fix(grok): Prefer a live credential over a stale keyring entry

[euxaristia]

Summary

Grok reported HTTP 401 and rendered as degraded with no usage windows, on a
machine where the Grok CLI was signed in and working.

load_grok() read the OS keyring before ~/.grok/auth.json and returned the
first entry it found regardless of freshness, so an expired cairn-code
oauth:xai token shadowed the live one the CLI maintains. The refresh probe
could not rescue it: grok models refreshes the CLI's file, nothing rewrites
the keyring copy, so the probe fired on every call, refreshed a file that was
then ignored, and handed back the same dead token.

This is a regression. Before cef4016 ("Load Grok and Claude tokens from
cairn-code keyring entries"), load_grok() read only the file. That commit put
the keyring ahead of the file for both Claude and Grok; 6f07976 ("Handle
nullable Claude quota fields and prioritize canonical credentials file") fixed
it one commit later for Claude alone.

The second commit closes a smaller gap in the same path: a billing call that
never landed was indistinguishable from a window that had gone unspent.

Changes

• src/credentials/mod.rs: load_grok() now collects every candidate with the
  canonical file first and picks the first still-live one via pick_live(),
  rather than the first that merely exists. A plain order swap would fix Grok
  while breaking the mirror case Antigravity documents, where the keyring holds
  the live token and the file is the stale artifact.
• src/fetch.rs: used_percent becomes an Option, reported as
  usage unavailable when billing does not answer. An absent
  creditUsagePercent inside a successful response stays a real zero, since
  the payload is protobuf JSON and omits fields still holding their default.

Test plan

• cargo fmt --check
• RUSTFLAGS="-Dwarnings" cargo clippy --all-targets
• cargo test (160 pass, up from 155)
• Three of the new tests fail against the old code, confirmed by reverting each
  change in place:
  • a_stale_keyring_entry_never_shadows_a_live_credential fails with
    left: "expired-keyring-copy", right: "refreshed-cli-token"
  • grok_billing_that_never_answers_reports_unavailable_rather_than_zero fails
    with left: "0% used", right: "usage unavailable"
• Manual: limits status -p grok --json returned
  "ErrorMessage": "Grok user API returned HTTP 401" with an empty Windows
  array before, and a healthy Weekly window with a reset countdown after.
• The reported percentage was cross-checked against the Grok CLI's own
  billing: fetched credits config log records, which show the field present
  with real values for the previous billing period and absent in the new one.

Fixes #36

[coderabbitai]

Caution

Review failed

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ℹ️ Recent review info

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📥 Commits

Reviewing files that changed from the base of the PR and between 9670d91 and a43655c.

📒 Files selected for processing (2)

• src/credentials/mod.rs
• src/fetch.rs

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📝 Walkthrough

Walkthrough

The change updates Grok credential selection to evaluate all supported stores and improves billing usage reporting. Fresh credentials now take precedence over stale entries. Failed billing requests now display unavailable usage instead of zero.

Changes

Grok handling

Layer / File(s)	Summary
Credential candidate selection src/credentials/mod.rs	load_grok aggregates canonical CLI and keyring credentials. It selects the first fresh token and retains the canonical token when all candidates are expired. Tests cover fresh, stale, and empty candidate sets.
Billing usage status src/fetch.rs	Grok billing treats omitted creditUsagePercent as zero and failed billing requests as unavailable. Tests cover omitted, failed, and successful billing responses.

Estimated code review effort: 3 (Moderate) | ~20 minutes

Poem

A rabbit checks each token in line,
Fresh keys hop forward, stale keys decline.
Zero means quiet, not data gone,
Unknown now speaks when calls move on.
Grok’s weekly gauge shines clear and bright.

🐇

✨ Finishing Touches

📝 Generate docstrings

• Create stacked PR
• Commit on current branch

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