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/agents/lead.md

@@ -140,9 +140,8 @@ Minimum viable workflow for well-understood follow-on work: **PLAN → EXECUTE
 - Memory: record reusable patterns in `.memory/decisions.md` under `## Retrospective: <topic>`.
 - **Librarian dispatch:** After significant feature work, dispatch `librarian` to:
   1. Update project documentation (README, docs/*)
-  2. Sync cross-tool instruction files (`AGENTS.md`, `CLAUDE.md`, `.github/copilot-instructions.md`, `.cursorrules`)
+  2. Update `.github/copilot-instructions.md` if project conventions/architecture changed
   3. Update `.memory/knowledge.md` with new architecture/pattern knowledge
-  4. Merge any knowledge from instruction files that other tools may have added
 
 ## Knowledge Freshness Loop
 
@@ -370,6 +369,6 @@ This is a ~30-second mental check, not a full review. If something looks obvious
 ## Documentation Completion Gate
 
 - For every completed project change set, documentation must be created or updated.
-- Minimum required documentation coverage: `README` + relevant `docs/*` files + cross-tool instruction files (`AGENTS.md`, `CLAUDE.md`, `.github/copilot-instructions.md`, `.cursorrules`) when project conventions, commands, architecture, workflow, policies, or agent behavior changes.
+- Minimum required documentation coverage: `README` + relevant `docs/*` files + `.github/copilot-instructions.md` when project conventions, commands, architecture, workflow, policies, or agent behavior changes.
 - **Documentation is a completion gate, not a follow-up task.** Do not declare a task done, ask "what's next?", or proceed to commit until doc coverage is handled or explicitly deferred by the user. Waiting for the user to ask is a failure.
 - **Always delegate to `librarian`** for documentation coverage checks and cross-tool instruction file maintenance. The librarian is the specialist — do not skip it or handle docs inline when the librarian can be dispatched.