feat(capacity): add Claude subscription usage

[codeaholicguy]

Summary

• add Claude subscription usage to the capacity command using the existing capacity contracts
• resolve OAuth credentials from the environment, the active Claude profile, or the default-profile macOS Keychain item
• map session, weekly, model-scoped, and optional extra-usage capacity without inventing token totals
• preserve sanitized provider errors and fixture-based coverage for auth, rate limits, and malformed or partial payloads

Validation

• npm test --workspace=@ai-devkit/agent-manager -- src/tests/capacity/claude.test.ts (27 tests)
• npm test --workspace=ai-devkit -- src/tests/commands/capacity/command.test.ts (13 tests)
• npm run build --workspace=@ai-devkit/agent-manager
• npm run build --workspace=ai-devkit
• pre-commit secret scan, repository lint, and full repository tests (2,307 tests)

Risks

• the OAuth usage endpoint and fixed Claude Code User-Agent may evolve upstream
• the first macOS Keychain fallback can trigger an operating-system access prompt
• no live account request or real Keychain access was performed during validation