Phase 6: Branching Prototype & Final Documentation

[rmax]

Description

Implement the candidate frontier and branching strategy, complete Mode D, generate comparison reports with branching, and finalize all documentation including ADRs, threat model, architecture diagrams, and limitations.

Key Deliverables

1. Candidate Frontier — search-frontier abstraction with add/choose/remove
2. Branching Strategy — Mode D branching after stagnation (max factor 2)
3. Branch Pruning — remove dominated or exhausted branches
4. Comparison Reporting — report with branching vs. linear comparison
5. Architecture Decision Records — 5 ADRs covering key decisions
6. Threat Model — comprehensive threat analysis and mitigations
7. Mermaid Diagrams — component architecture, state machine, execution flow
8. Final Documentation — all docs/ files complete and accurate

Acceptance Criteria

• Frontier correctly adds and selects candidates
• Branching creates at most 2 concurrent branches
• Pruning removes dominated candidates
• Mode D completes with branching strategy
• ADRs cover: ADK integration, Git state, evaluation vectors, SQLite, Docker sandbox
• Threat model documents all 18+ threats with mitigations
• mkdocs build --strict succeeds with all diagrams
• All 6 phases are documented in progress/status
• uv run pytest passes (>85% coverage)
• make ci-check passes end-to-end

Depends on: #3, #4, #5, #6