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/explorer.md

@@ -20,12 +20,12 @@ Purpose:
 
 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 tools to gather architecture context.
 3. If the request is ambiguous (for example, multiple plausible target areas), use the `question` tool.
 4. Do not write files or execute shell commands.
 5. Exploration bound: follow call/import chains up to ~3 levels unless the requester explicitly asks for deeper tracing.
-6. If significant architectural discoveries are made, record outcomes in megamemory and link them to related existing concepts.
+6. If significant architectural discoveries are made, record outcomes in the relevant `.memory/` files and link related sections with markdown cross-references.
 7. Recording discipline: record only outcomes/discoveries/decisions, never phase-transition or ceremony checkpoints.
 
 Required output contract (required):