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/skills/doc-coverage/SKILL.md

@@ -21,9 +21,14 @@ For every completed change set, verify documentation coverage:
 - [ ] Do new features need their own doc page?
 - [ ] Are API changes reflected in API documentation?
 
-### 3. AGENTS.md / agent definitions
-- [ ] Did this change alter workflow, policies, or agent behavior?
-- [ ] If yes, update AGENTS.md or the relevant agent definition file.
+### 3. Agent/Tool Instruction Files
+- [ ] Did this change alter workflow, policies, conventions, commands, or architecture guidance?
+- [ ] `AGENTS.md` (OpenCode instructions) is updated if needed.
+- [ ] `CLAUDE.md` (Claude Code instructions) is updated if needed.
+- [ ] `.github/copilot-instructions.md` (GitHub Copilot instructions) is updated if needed.
+- [ ] `.cursorrules` (Cursor instructions) is updated if needed.
+- [ ] Are all instruction files consistent with each other?
+- [ ] Do instruction files reflect the current project state?
 
 ### 4. Inline documentation
 - [ ] Are complex functions/components documented with comments explaining **why**, not **what**?