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/status.md

@@ -1,14 +1,16 @@
 ---
-description: Show persistent cross-session work status from megamemory
+description: Show persistent cross-session work status from .memory files
 agent: lead
 ---
 
-Load persistent project status from megamemory and provide a resume-oriented status report.
+Load persistent project status from `.memory/` and provide a resume-oriented status report.
 
 Requirements:
 
-- Call `megamemory:list_roots` to load top-level concepts.
-- Call `megamemory:understand` for in-progress work, open decisions, and pending next steps.
+- Read the `.memory/` directory listing first.
+- Read `.memory/plans/*.md` for in-progress and recently completed work.
+- Read `.memory/decisions.md` for pending or unresolved decisions.
+- If present, use `.memory/knowledge.md` for additional context.
 - Summarize:
   1. What is currently in progress
   2. What was completed recently