Add codexbar guard CLI subcommand

[Finesssee]

Summary

Ports upstream codexbar guard for Windows automation quota gates.

• codexbar guard --provider <id> checks remaining quota on session (primary) or weekly (secondary) windows
• Stable exit codes: 0 ok, 1 blocked (below threshold), 64 invalid args, 69 unavailable
• --min-remaining (default 10), --window session|weekly, --timeout, --json/--pretty, --fail-open
• Bounded fetch through existing instantiate_provider + fetch_usage paths
• Unit tests for pure exit-code mapping, headroom, arg validation, and timeout

Invoke

cargo run -p codexbar -- guard --provider claude
cargo run -p codexbar -- guard --provider claude --min-remaining 20 --window weekly
cargo run -p codexbar -- guard --provider codex --json --pretty
cargo run -p codexbar -- guard --provider claude --fail-open

Test plan

• cargo test --manifest-path rust/Cargo.toml --lib guard (18 passed)
• Manual: codexbar guard --provider <configured> with live credentials

No version bump.

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