fix(keychain): include file store account alongside keychain entries

[itsahedge]

What & Why

On macOS, readAllClaudeAccounts() treats the keychain and ~/.claude/.credentials.json as either/or: if any keychain service matching Claude Code-credentials* parses, the credentials file is never included in the account list. A stale keychain entry (for example one left behind by an older Claude Code version that no longer maintains it) can therefore hide a perfectly healthy file store completely.

In that state the fallback machinery has nothing to work with. When the stale entry's refresh gets rate-limited, tryFallbackAccount finds no sibling account, the refresh-cooldown path returns credentials_unavailable, and a long-lived process (such as an opencode serve instance) fails every request with "Claude token refresh is rate-limited; retry shortly" while a valid token sits unused in the file, indefinitely. Observed in production: plugin_init showed a single suffixed keychain account, the file store held a token valid for hours, and the process wedged until manually restarted.

This PR includes the file store as its own account alongside keychain entries, so the existing borrow/fallback paths can rescue a wedged keychain account with the file token. Keychain-first ordering is unchanged, and the file account is skipped when a keychain account already carries the same access token, which keeps the account list identical in the common case where Claude Code writes both stores.

Changes

• readAllClaudeAccounts() appends a source: "file" account (with its config dir and email) after keychain accounts, unless a keychain account already holds the same access token.
• Test: a usable keychain entry plus a credentials file with a different token now yields both accounts, with the file account carrying the correct credentials and config dir.
• Test: a keychain entry whose token matches the file store yields a single account (no duplicate).

[greptile-apps]

Your trial has ended. Reactivate Greptile to resume code reviews.