release: v7.2.0 — agent-setup clarity, single-source exports, adoption docs

[rennf93]

Summary

Release-prep version bump 7.1.1 → 7.2.0 for the work already merged: #92 (O1/O2 agent-init clarity + [agent] extra), #96 (O3 export single-source-of-truth), #94 (O7 Passive-to-Active guide), #95 (O8 behavioral wiring docs).

• make bump-version VERSION=7.2.0 → pyproject, .mike.yml, versions.json, docs/index.md example tag.
• CHANGELOG.md + docs/release-notes.md v7.2.0 entry filled (was the bump scaffold), in the repo's inline - **Added** — style (setext-consistent — pymarkdown MD003).

Minor bump — all changes additive/backward-compatible. Requires guard-core 3.2.0+ at runtime (uses AgentPackageNotInstalledError); dependency intentionally left unpinned per project convention. Tag v7.2.0 after merge — but only after guard-core 3.2.0 publishes to PyPI, or CI/install will fail to resolve the new symbol.