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**?

.config/opencode/skills/git-workflow/SKILL.md

@@ -12,8 +12,8 @@ description: Procedures for git commits, worktrees, branches, and GitHub PRs —
    - Focus on **why**, not **what**.
    - 1-2 sentences max.
    - Match the repository's existing style.
-5. Check for secrets: do NOT commit `.env`, credentials, key files, or `.megamemory/` contents.
-6. If `.megamemory/` is not in `.gitignore`, add it before the first commit.
+5. Check for secrets: do NOT commit `.env`, credentials, or key files.
+6. The `.memory/` directory IS committed to git (it is meant for collaboration).
 7. Stage relevant files: `git add <files>` (not blindly `git add .`).
 8. Commit: `git commit -m "<message>"`.
 9. Run `git status` after commit to verify success.

.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