Version 0.3.0: resolver/merge/cache refactor, signing design notes

- Split the monolith into package/provider/fetch/merge/cache modules with
  the Resolver deciding install/adopt/update/merge/conflict outcomes
- Three-way merges via git merge-file/diff3 against a content-addressed
  merge-base cache; conflicts go to a .picopackage-merge sibling
- Tests for package, provider, resolver, merge, and cache
- notes.md: update UX and signing design — diff-by-default, SSH signature
  identity pinning (TOFU), key changes as a hard stop, exit-code contract
- Add CLAUDE.md; remove the pre-refactor exe/pppkg monolith and scratch files

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01EiyJC73Mz8xZyCTvCEY8qn

lib/picopackage.rb

@@ -1,18 +1,20 @@
 # frozen_string_literal: true
 
 require_relative "picopackage/version"
+require_relative "picopackage/package"
+require_relative "picopackage/cache"
+require_relative "picopackage/merge"
 require_relative "picopackage/fetch"
 require_relative "picopackage/provider"
-require_relative "picopackage/package"
 require_relative "picopackage/scanner"
 require_relative "picopackage/cli"
 
 module Picopackage
   class Error < StandardError; end
 
-  class FileTooLargeError < StandardError; end
+  class FileTooLargeError < Error; end
 
-  class FetchError < StandardError; end
+  class FetchError < Error; end
 
-  class LocalModificationError < StandardError; end
+  class LocalModificationError < Error; end
 end