feat: replace megamemory with markdown-based memory system

Remove the megamemory MCP knowledge graph and replace with plain
markdown files in .memory/ for tracking plans, research, knowledge,
and implementation state. This enables collaboration across people
and agentic coding tools (Claude Code, Copilot, Cursor, etc.).

Changes:
- Remove megamemory MCP from opencode.jsonc
- Delete tool/megamemory.ts and .megamemory/ database
- Rewrite all 25 config files to use .memory/ markdown files
- Add cross-tool instruction file awareness (AGENTS.md, CLAUDE.md,
  copilot-instructions.md, .cursorrules)
- Update save-memory, bootstrap-memory, status commands for md workflow
- Update all agent files, skills, and commands consistently

.config/opencode/agents/reviewer.md

@@ -26,7 +26,7 @@ Pipeline position:
 
 Operating rules:
 
-1. Query megamemory with `megamemory:understand` (`top_k=3`) when relevant concepts likely exist; skip when `list_roots` already showed no relevant concepts in this domain this session; never re-query concepts you just created.
+1. Read relevant `.memory/*.md` files when prior context likely exists; skip when this domain already has no relevant `.memory/` entries this session.
 2. Use read-only analysis; do not edit files or run shell commands.
 3. If review criteria are unclear, use the `question` tool.
 4. Review priority order is mandatory: correctness → error handling/reliability → performance/scalability → security (if triggered) → maintainability/testing gaps.
@@ -145,8 +145,8 @@ Output quality requirements:
 - Use exact `[file:line]` for every CRITICAL/WARNING item.
 - Keep `NEXT` as explicit fix actions, not generic advice.
 
-Megamemory duty:
+Memory file duty:
 
-- After issuing a verdict, record it in megamemory as a `decision` concept.
-- Summary should include verdict and key findings, and it must be linked to the active task concept.
+- After issuing a verdict, record it in `.memory/decisions.md` as a markdown section.
+- Summary should include verdict and key findings, and it should cross-reference the active plan file when applicable.
 - Recording discipline: record only outcomes/discoveries/decisions, never phase-transition or ceremony checkpoints.