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/commands/review.md

@@ -11,7 +11,7 @@ $ARGUMENTS
 
 Requirements:
 
-- Query megamemory first for project conventions and prior decisions.
+- Read relevant `.memory/*.md` files first for project conventions and prior decisions.
 - If review scope is ambiguous, ask one focused clarification question using the question tool.
 - Categorize findings: critical, warning, suggestion.
 - Include concrete file references and rationale.