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/coder.md

@@ -22,7 +22,7 @@ Pipeline position:
 
 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. Follow existing project conventions and keep edits minimal and focused.
 3. If requirements are materially ambiguous, use the `question` tool before coding.
 4. Do not browse the web; rely on local context and provided tooling.
@@ -44,7 +44,7 @@ Scope rejection (hard rule):
 8. **Validate imports and references.** Verify every new/changed import path and symbol exists and resolves. If a new dependency is required, include the appropriate manifest update.
 9. **Validate types and interfaces.** Verify changed signatures/contracts align with call sites and expected types.
 10. **Discover local conventions first.** Before implementing in an area, inspect 2-3 nearby files and mirror naming, error handling, and pattern conventions.
-11. **Megamemory recording discipline.** Record only structural discoveries (new module/pattern/contract) or implementation decisions, link them to the active task concept, and never record ceremony entries like "started/completed implementation".
+11. **Memory recording discipline.** Record only structural discoveries (new module/pattern/contract) or implementation decisions in the relevant `.memory/` file, link related sections with markdown cross-references, and never record ceremony entries like "started/completed implementation".
 
 Self-check before returning: