feat(credentials): Load Grok and Claude tokens and handle nullable quota fields

[euxaristia]

Summary

Loads Grok (xAI) and Claude OAuth credentials from OS credential stores and canonical files, and handles nullable Claude quota response fields.

Changes

• Updated keyring credential reading on Windows to probe {account}.{service} target formats and transparently decode UTF-16LE password blobs.
• Updated credentials loaders to inspect the OS keyring for cairn-code Grok entries and prioritized ~/.claude/.credentials.json for active Claude Code sessions.
• Updated Claude parser Bucket fields (resets_at, utilization) to Option<...> to survive null JSON values from Anthropic.
• Added unit tests for parsing Grok OAuth payloads, Grok CLI map payloads, nested/direct Claude OAuth structures, and nullable Claude response fields.

Refs #28
Fixes #28

Test plan

• Ran cargo fmt --check (clean)
• Ran cargo clippy --all-targets --all-features -- -D warnings (clean)
• Ran cargo test (155 tests passing)
• Ran cargo run -- status and verified live tracking across Claude, Grok, Antigravity, OpenCode Go, Copilot, and Codex

[euxaristia]

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📥 Commits

Reviewing files that changed from the base of the PR and between 649a95f and 8d8b680.

📒 Files selected for processing (3)

• src/credentials/keyring.rs
• src/credentials/mod.rs
• src/parsers/claude.rs

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