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/skills/doc-coverage/SKILL.md

@@ -21,14 +21,27 @@ For every completed change set, verify documentation coverage:
 - [ ] Do new features need their own doc page?
 - [ ] Are API changes reflected in API documentation?
 
-### 3. Agent/Tool Instruction Files
-- [ ] Did this change alter workflow, policies, conventions, commands, or architecture guidance?
-- [ ] `AGENTS.md` (OpenCode instructions) is updated if needed.
-- [ ] `CLAUDE.md` (Claude Code instructions) is updated if needed.
-- [ ] `.github/copilot-instructions.md` (GitHub Copilot instructions) is updated if needed.
-- [ ] `.cursorrules` (Cursor instructions) is updated if needed.
-- [ ] Are all instruction files consistent with each other?
-- [ ] Do instruction files reflect the current project state?
+### 3. Instruction File
+
+Check `.github/copilot-instructions.md` and its symlinks:
+
+- Does `.github/copilot-instructions.md` exist and contain current project info?
+- Do symlinks exist and point correctly?
+  - `AGENTS.md -> .github/copilot-instructions.md`
+  - `CLAUDE.md -> .github/copilot-instructions.md`
+  - `.cursorrules -> .github/copilot-instructions.md`
+- Does the instruction file contain:
+  - Project purpose and overview
+  - Tech stack and architecture
+  - Coding conventions
+  - Build/test/lint commands
+  - Project structure
+- Is the instruction file in sync with current project state?
+
+**Anti-patterns:**
+- Symlinks missing or pointing to wrong location
+- Instruction file is stale or empty
+- Instruction file duplicates `.memory/` tracking content (plans, research)
 
 ### 4. Inline documentation
 - [ ] Are complex functions/components documented with comments explaining **why**, not **what**?