Add handoff documentation

Overview, architecture notes, and next steps for anyone
picking this up — including Rails API gotchas discovered
during implementation.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

handoff/overview.md

@@ -0,0 +1,32 @@
+# Whetrails — Project Handoff
+
+## What this is
+
+A Ruby gem that makes ActiveRecord callback and validation chains visible to developers.
+
+When a Rails developer calls `.save` on a model, Rails fires a chain of callbacks and validations in a specific order. These are registered in scattered places (the model file, concerns, gems) and Rails never surfaces the full picture in one place. Whetrails extracts and displays that chain: what fires, in what order, where it's defined, and under what conditions.
+
+This is the first half of a two-part tool. The second half (not yet built) uses the extracted chain as input to a test coverage gap analyzer — showing which callbacks and validators are never exercised by the test suite.
+
+## Current state (as of 2026-06-08)
+
+Two layers are complete:
+
+1. **Core inspector** — extracts the full chain from any ActiveRecord model class at runtime
+2. **CLI** — `whetrails inspect <ModelName>` boots the host Rails app and renders the chain to the terminal
+
+The next layer (Rails engine / browser UI) has not been started.
+
+## Build order
+
+```
+[DONE]  1. Gem core       lib/whetrails/inspector.rb
+[DONE]  2. CLI            exe/whetrails + lib/whetrails/cli.rb
+[NEXT]  3. Rails engine   browser UI mounted at /whetrails
+        4. Test gap layer  uses chain data to find uncovered callbacks/validators
+```
+
+## Repository
+
+- Gitea: http://localhost:3000/bill/whetrails
+- Branch: main