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/bootstrap-memory.md

@@ -60,26 +60,13 @@ constraints). Include rationale when known.
 - If research artifacts are discovered, create `.memory/research/<topic>.md`
   with findings and references.
 
-## Step 6: Delegate instruction file creation to librarian
+## Step 6: Update instruction file
 
-After `.memory/knowledge.md` is populated, dispatch the `librarian` subagent to
-create or update all cross-tool instruction files from `.memory/knowledge.md`:
+If `.github/copilot-instructions.md` exists, update it with any new project knowledge
+discovered during bootstrap (architecture, conventions, commands). The symlinks
+(`AGENTS.md`, `CLAUDE.md`, `.cursorrules`) will automatically reflect changes.
 
-- `AGENTS.md`
-- `CLAUDE.md`
-- `.github/copilot-instructions.md`
-- `.cursorrules`
-
-Each file should include the same core project guidance:
-- project overview and purpose
-- tech stack
-- architecture summary
-- coding conventions/patterns
-- build/test/lint commands
-- project structure overview
-
-If any of these files already exist, preserve existing project-specific content
-and merge in new knowledge instead of overwriting wholesale.
+If the instruction file doesn't exist, note that `/init` should be run to create it.
 
 ## Guidelines