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/work-decomposition/SKILL.md

@@ -106,7 +106,7 @@ When triggering a checkpoint, present:
 Do not interrupt the user for:
 - Implementation details (naming, file organization, code patterns)
 - Choices fully determined by existing codebase conventions
-- Decisions already covered by prior user answers or megamemory guidance
+- Decisions already covered by prior user answers or cached guidance in `.memory/decisions.md`
 
 ## Coder Dispatch Rules
 
@@ -132,7 +132,7 @@ Each coder dispatch MUST include:
 2. **Specific file paths and edit points** from discovery (not vague references)
 3. **Discovered values verbatim**: i18n keys, API signatures, component names, existing patterns
 4. **Worktree path** for all file operations
-5. **Active megamemory concept ID** for the task
+5. **Active plan file path** for the task (for example: `.memory/plans/<feature>.md`)
 6. **Quality tier** so coder understands the expected rigor
 
 ### Anti-patterns — Never Do These