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/commands/save-memory.md

@@ -55,18 +55,11 @@ Example cross-reference:
 Re-read updated `.memory/` files to confirm they reflect current understanding.
 Report what was added/updated and where.
 
-## Step 6: Delegate instruction file sync to librarian
+## Step 6: Update instruction file if project knowledge changed
 
-If `.memory/knowledge.md` was **materially** updated in this save (architecture,
-conventions, commands — not just plans/research), dispatch the `librarian`
-subagent to:
-
-1. Read all cross-tool instruction files (`AGENTS.md`, `CLAUDE.md`,
-   `.github/copilot-instructions.md`, `.cursorrules`)
-2. Check for knowledge in instruction files not yet in `.memory/knowledge.md`
-   and merge it inward
-3. Update instruction files to reflect new `.memory/knowledge.md` content
-4. Report sync status
+If `.memory/knowledge.md` was **materially** updated (architecture, conventions,
+commands — not just plans/research), update `.github/copilot-instructions.md` to
+reflect the changes. The symlinks automatically propagate to all tools.
 
 If updates were only plans/research/tracking details, skip this step.