Prepare Agent Code Guard 0.2.0

[stef-k]

Summary

• bump the authoritative distribution version to 0.2.0
• finalize the six-feature 0.2.0 changelog while retaining Unreleased
• add first-use human and agent onboarding, the recommended manual/hook workflow, and precise skill activation guidance
• clarify completed-result, argparse, and other tool-error exit behavior

Validation

• source suite: 305 tests passed
• compileall and diff checks passed
• installed-console zero-baseline self-analysis passed with visible REVIEW findings
• fresh wheel and sdist passed Twine, archive inspection, isolated installation, doctor, version, JSON, skill, scope, and LOC-ratchet probes
• final two-line contract remediation was documentation-only and inspected directly

Part of #111