feat: adopt symlink approach for cross-tool instruction files

Replace 4 separate instruction files with 1 real file + symlinks:
- .github/copilot-instructions.md is the canonical instruction file
- CLAUDE.md and .cursorrules are symlinks to it
- AGENTS.md stays as global config (not a project instruction file)

This eliminates all sync/merge logic - changes propagate automatically.

Changes:
- AGENTS.md: rewrite Cross-Tool Instruction Files section for symlink convention
- librarian.md: simplify to maintain single instruction file + verify symlinks
- lead.md: simplify PHASE-WRAP and Documentation Completion Gate
- commands (init, bootstrap-memory, save-memory): update for symlink model
- doc-coverage skill: verify symlinks exist and point correctly

.config/opencode/.memory/knowledge.md

@@ -6,7 +6,7 @@ This is the OpenCode agentic configuration for a multi-agent coding workflow. It
 - **Config file:** `opencode.jsonc` — defines default agent, plugins, global permissions, and MCP servers.
 - **Agent definitions:** `agents/*.md` — one file per agent with YAML frontmatter (model, permissions, temperature) and markdown instructions.
 - **Memory system:** `.memory/` — persistent project knowledge, decisions, plans, and research in markdown.
-- **Cross-tool instruction files:** `AGENTS.md`, `CLAUDE.md`, `.github/copilot-instructions.md`, `.cursorrules` — derived views of project knowledge for different agentic tools.
+- **Cross-tool instruction files:** `.github/copilot-instructions.md` is the single real instruction file; `AGENTS.md`, `CLAUDE.md`, and `.cursorrules` should be symlinks to it.
 
 ## Agent Architecture
 
@@ -32,6 +32,13 @@ Agents with memory-only edit access are constrained by instruction-level guardra
 
 Tiered pipeline (Tier 1/2/3) defined in `agents/lead.md`. Standard flow: `explorer/researcher → coder → reviewer → tester → librarian`.
 
+## Documentation Skill Conventions
+
+- `skills/doc-coverage/SKILL.md` section `### 3. Instruction File` validates one canonical instruction file (`.github/copilot-instructions.md`) plus symlink correctness for `AGENTS.md`, `CLAUDE.md`, and `.cursorrules`, rather than treating all four as independently edited files.
+- The checklist explicitly flags duplication of `.memory/` tracking content (plans/research) inside instruction files as an anti-pattern.
+
+See [Cross-Tool Instruction Files](../AGENTS.md#cross-tool-instruction-files) and [Doc coverage checklist section 3](../skills/doc-coverage/SKILL.md#3-instruction-file).
+
 ## MCP Servers
 
 - **Context7:** Remote documentation lookup (`https://mcp.context7.com/mcp`)