refactor: Replace CachedWorkflowState with minimal instruction buffer

Remove redundant 7-field CachedWorkflowState that duplicated ConversationManager
state and could get out of sync (e.g. on session resume). Replace with:

- BufferedInstructions: minimal buffer set by proceed_to_phase/start_development,
  consumed+cleared by chat.message hook. Falls back to WhatsNextHandler when empty.
- getWorkflowState(): reads from shared ServerContext's ConversationManager for
  tool.execute.before and compaction hooks (no more throwaway instances).

Net -121 lines. All tests pass.

Author: mrsimpson

.vibe/development-plan-remove-internal-plugin-state.md

@@ -0,0 +1,53 @@
+# Development Plan: responsible-vibe (remove-internal-plugin-state branch)
+
+*Generated on 2026-03-31 by Vibe Feature MCP*
+*Workflow: [epcc](https://mrsimpson.github.io/responsible-vibe-mcp/workflows/epcc)*
+
+## Goal
+Remove redundant `CachedWorkflowState` from the opencode plugin. The plugin maintains internal state (`cachedState`) that duplicates what `ConversationManager` already persists to disk. This state gets out of sync on session resume. Replace all reads from `cachedState` with reads from `ConversationManager` via `getServerContext()`.
+
+## Key Decisions
+1. **`ProceedToPhaseHandler` already persists to disk** (line 125-127 of mcp-server proceed-to-phase.ts calls `conversationManager.updateConversationState`), so the next `WhatsNextHandler` call in `chat.message` will read correct state. The "race condition" from commit 8985941 does not actually exist — the tool writes to disk before returning.
+2. **Three consumers of `cachedState`**: `chat.message` (instructions), `tool.execute.before` (file patterns), `experimental.session.compacting` (phase info). All can read from `ConversationManager` instead.
+3. **`updateCachedState` callback passed to tool handlers** can be removed — tools no longer need to update plugin state.
+4. **`loadStateDirectly()` creates its own `FileStorage`/`ConversationManager`** — wasteful. Use the shared `ServerContext` instead.
+5. **Replace 7-field CachedWorkflowState with minimal instruction buffer**: `lastPhaseInstructions: { phase: string, instructions: string, allowedFilePatterns: string[] } | null`. Set by proceed_to_phase/start_development, consumed+cleared by chat.message. Falls back to WhatsNextHandler if null.
+6. **tool.execute.before and compacting**: Read from ConversationManager via shared ServerContext. No more `loadStateDirectly` with throwaway instances.
+7. **Tool handlers**: Replace `updateCachedState` callback with simpler `setLastInstructions` callback.
+
+## Notes
+- `cachedState` is used in 4 places: `chat.message`, `tool.execute.before`, `experimental.session.compacting`, and initial load at plugin startup
+- Tool handlers `start_development` and `proceed_to_phase` accept `updateCachedState` callback — needs removal from signature
+- The `ServerContext` is already cached and reused (`cachedServerContext`), so reading from it is cheap
+- `ConversationManager.getConversationContext()` reads from `FileStorage` (disk) every call — no in-memory cache to go stale
+- `FileStorage.initialize()` only creates directories — the shared ServerContext's FileStorage works because the directory already exists from previous operations
+- Minor pre-existing issue: `initializeServerContext` creates a throwaway `FileStorage` — out of scope
+
+## Explore
+<!-- beads-phase-id: responsible-vibe-23.1 -->
+### Tasks
+
+*Tasks managed via `bd` CLI*
+
+## Plan
+<!-- beads-phase-id: responsible-vibe-23.2 -->
+### Tasks
+
+*Tasks managed via `bd` CLI*
+
+## Code
+<!-- beads-phase-id: responsible-vibe-23.3 -->
+### Tasks
+
+*Tasks managed via `bd` CLI*
+
+## Commit
+<!-- beads-phase-id: responsible-vibe-23.4 -->
+### Tasks
+
+*Tasks managed via `bd` CLI*
+
+
+
+---
+*This plan is maintained by the LLM and uses beads CLI for task management. Tool responses provide guidance on which bd commands to use for task management.*