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/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.