[WIP] feat: Secrets management (keyring + redaction)

[quangdang46]

Gap #26 — Medium

Dedicated secrets manager with keyring integration and automatic redaction.

Source: codex

What's needed:

• SecretsManager with pluggable backends
• Keyring-backed storage (OS keychain integration)
• Scoped secrets: Global and Environment(String) scopes
• redact_secrets() sanitizer for scrubbing from output
• Environment ID derived from git repo root or cwd hash

Reference: codex codex-rs/secrets/