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/opencode.jsonc

@@ -39,11 +39,6 @@
 				"chromium"
 			],
 			"enabled": true
-		},
-		"megamemory": {
-			"type": "local",
-			"command": ["megamemory"],
-			"enabled": true
 		}
 	}
 }