feat: wire librarian as the primary agent for documentation and instruction file maintenance

Expand librarian role to own cross-tool instruction file sync,
.memory/ file maintenance, and knowledge merge workflows. Lead
now proactively dispatches librarian in PHASE-WRAP and Documentation
Completion Gate instead of handling docs inline.

.config/opencode/agents/lead.md

@@ -138,6 +138,11 @@ Minimum viable workflow for well-understood follow-on work: **PLAN → EXECUTE
   - What was tricky
   - What patterns should be reused
 - 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`)
+  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
 
@@ -367,4 +372,4 @@ This is a ~30-second mental check, not a full review. If something looks obvious
 - 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.
 - **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.
-- Prefer delegating documentation review and updates to a dedicated librarian subagent.
+- **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.