docs: add docs/architecture.md (sprint 0 mirror)
High-level architecture snapshot reflecting feature/backend-skeleton
@ 12d131c and feature/frontend-skeleton @ b505a65. Covers:
- Repo + backend + frontend module trees.
- §8 aggregates with delta annotations vs the frozen spec.
- F11 permission matrix mapping to rbac/matrix.py.
- Auth split (RT bcrypt session vs SOC opaque token) per D-003 / D-006.
- Cleanup templating (Jinja sandbox + regex_extract D-011 semantics).
- C2 abstraction layer + Mythic / Home stub.
- Storage pools layout (CAS blobs + flat evidence) per D-012.
- Sprint 0 happy-path flow + post-sprint scope boundary.
- Known WARN items (audit chain unverified, scope on /engagements,
role free-text on engagement_member, deferred Q-003..Q-005).
- Anticipated-vs-v2 table summarising D-004 / D-008 / D-012 / D-013.
This is a living mirror — when code disagrees, code wins, file a doc fix.
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