Add provider availability to JSON output

[jacobcxdev]

Summary

• Add provider-level availability metadata to cq --json with state, guidance, reason, remaining quota, and reset horizon.
• Preserve account-level active compatibility while documenting that agents should use provider availability instead.
• Cover limited/exhausted/unknown/error and active-account edge cases, including exhausted status overriding residual window values.

Test plan

• go test -race -count=1 ./...
• go vet ./...
• go run ./cmd/cq --json