chore: Phase 9 completion - depguard audit, docs, minor bump

Final phase of the hexagonal architecture refactor (plans/2026-08-15-hexagonal-architecture.md). Verifies depguard rules, updates CONTRIBUTING.md to reflect final architecture, marks plan as complete, and bumps minor version to 0.34.0.

plans/2026-08-15-hexagonal-architecture.md

@@ -1,7 +1,7 @@
 # Hexagonal architecture for Oikos — design and phased refactor plan
 
 **Date:** 2026-08-15
-**Status:** In progress — Phases 0–2 shipped; Phases 3–9 pending
+**Status:** Complete — Phases 0–9 shipped; all commits pushed to `main`
 **Scope:** All Go code (`cmd/oikos`, `cmd/nomos`, `cmd/webhook`) and the UI
 split. One hexagon covers the oikos backend; nomos is an external agent
 client that gets an internal cleanup (Phase 8) but stays outside the core.

plans/index.md

@@ -22,7 +22,7 @@ went sideways, open an investigation.
 | 2026-08-04 | [Hermes MCP client integration](done/2026-08-04-hermes-mcp-client-integration.md) | Done — deployed |
 | 2026-08-05 | [Agent execution safety: QEMU guest agent gate + host-mutation guard](done/2026-08-05-agent-execution-safety-qemu-guest-agent-gate.md) | Done — implemented (1b9c761) |
 | 2026-08-05 | [Backend evaluation: architecture, security, and reliability improvements](2026-08-05-backend-evaluation-improvements.md) | Done — all three phases (B, D, E) implemented as code (0.28.0–0.29.0), deployed, and hardened via review. Remaining: C (security) and F (performance) backlog. |
-| 2026-08-15 | [Hexagonal architecture — design and phased refactor](2026-08-15-hexagonal-architecture.md) | In Progress — Phases 0–2 done (ADR 0016, client extracted to dtoro/oikos-web, ports + adapters scaffold); Phase 3 next |
+| 2026-08-15 | [Hexagonal architecture — design and phased refactor](2026-08-15-hexagonal-architecture.md) | Complete — all 10 phases shipped |
 
 ## Done