docs: remove obsolete implementation tracker

AGENTS.md

@@ -16,25 +16,23 @@
 
 - Do not duplicate existing GitHub issues.
 - Keep issue descriptions current when scope or reproduction steps change.
-- Store AI-generated planning documents in `history/`, not the repository root;
-  the persistent root `IMPLEMENTATION.md` tracker described below is the exception.
+- Store AI-generated planning documents in `history/`, not the repository root.
 - **ALWAYS run `coderabbit --plain` before committing** to catch issues early.
 - All commits must use the `taliesin-ai` identity.
 - Never push until the user gives explicit manual confirmation.
 
 ### Managing AI-Generated Planning Documents
 
 AI assistants often create planning and design documents during development:
-- PLAN.md, IMPLEMENTATION.md, ARCHITECTURE.md
+- PLAN.md, ARCHITECTURE.md
 - DESIGN.md, CODEBASE_SUMMARY.md, INTEGRATION_PLAN.md
 - TESTING_GUIDE.md, TECHNICAL_DESIGN.md, and similar files
 
 **Best Practice: Use a dedicated directory for these ephemeral files**
 
 **Recommended approach:**
 - Create a `history/` directory in the project root
-- Store all ephemeral AI-generated planning/design docs in `history/`;
-  keep the persistent root `IMPLEMENTATION.md` tracker in place
+- Store all ephemeral AI-generated planning/design docs in `history/`
 - Keep the repository root clean and focused on permanent project files
 - Only access `history/` when explicitly asked to review past planning
 
@@ -87,35 +85,6 @@ measured bug they prevent.
   implementation, follow `docs/src/content/docs/guides/streams-from-websockets.mdx` —
   a mechanical checklist, including the differences that break silently.
 
-## Implementation Tracking (IMPLEMENTATION.md)
-
-**IMPORTANT**: The `IMPLEMENTATION.md` file at the repository root is a **persistent tracking document** for the GTK4 / WebKitGTK 6.0 / GTK3-legacy implementation work. It is NOT an ephemeral planning document.
-
-As of 2026-05-16 (issue #5459), GTK4 + WebKitGTK 6.0 is the **default** Linux stack; GTK3 + WebKit2GTK 4.1 is a legacy opt-in (`-tags gtk3`) for one v3 cycle and is scheduled for removal in v3.1. The default-flip rationale is recorded in `IMPLEMENTATION.md` Decision 1.1.
-
-### Requirements
-
-1. **Update with EVERY commit** that touches GTK4/WebKitGTK 6.0 or legacy GTK3 code
-2. **Track all architectural decisions** with context, decision, and rationale
-3. **Maintain progress status** for each implementation phase
-4. **Document API differences** between the GTK4 default and GTK3 legacy paths
-5. **Keep file references** accurate and up-to-date
-
-### What to Update
-
-- Phase completion status (✅ COMPLETE, 🔄 IN PROGRESS, 📋 PENDING)
-- New decisions made during implementation
-- Files created or modified
-- Changelog entries with dates
-- TODO items discovered during work
-
-### Commit Message Pattern
-
-When updating IMPLEMENTATION.md:
-```
-docs: update implementation tracker for [phase/feature]
-```
-
 ## Landing the Plane (Session Completion)
 
 **When ending a work session**, complete the applicable steps below. Never push without explicit manual confirmation from the user.