docs: add architecture and workflow diagrams

[saintdle]

Summary

• add a top-level Mermaid component map showing Agentflow, Beads, provider lanes, workspaces, results, and Git/GitHub
• add a persistent-controller lifecycle diagram covering approval, preflight, claims, review, remediation, halts, and completion
• add a security trust-boundary diagram for controller authority, task-scoped workers, external providers, return contracts, and integration
• add regression tests for diagram presence, critical paths, and balanced Mermaid fences

Visual verification

• rendered all three diagrams with pinned Mermaid CLI 11.12.0
• completed two visual passes
• reflowed the README diagram vertically and humanized lifecycle labels after the first pass
• confirmed no connector crosses a node or obscures another label

Validation

• full suite: 453 tests passed, 1 skipped
• python3 scripts/validate.py
• git diff --check

Beads: agentflow-aot

This PR will not merge itself.