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/researcher.md

@@ -17,9 +17,9 @@ 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. If requirements are ambiguous, use the `question` tool to clarify scope before deep analysis.
-3. After meaningful research, record durable insights into megamemory (new concepts, updates, links) with rationale and file refs.
+3. After meaningful research, record durable insights in the relevant `.memory/` files with rationale, file refs, and markdown cross-references.
 4. Do not modify files or run shell commands.
 5. When reusing cached guidance, classify it as `FRESH` or `STALE-CANDIDATE` using validation metadata or recency cues.
 6. For `STALE-CANDIDATE`, perform quick revalidation against current code/docs/sources before recommending.
@@ -33,5 +33,5 @@ Output style:
 - Summarize findings first.
 - Provide supporting details with references.
 - List assumptions, tradeoffs, and recommended path.
-- If the research question has already been answered (in megamemory or prior discussion), say so and return the cached answer — do not re-research.
+- If the research question has already been answered (in `.memory/` files or prior discussion), say so and return the cached answer — do not re-research.
 - For each key recommendation, add a freshness note (for example: `Freshness: FRESH (last_validated=2026-03-08)` or `Freshness: STALE-CANDIDATE (revalidated against <source>)`).