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

.standard.yml

@@ -0,0 +1,9 @@
+# standardrb reads this, not .rubocop.yml — keep the two in step.
+ruby_version: 3.1
+
+ignore:
+  # Fixtures are deliberately malformed or copied verbatim from elsewhere;
+  # linting them defeats the point of having them.
+  - "test/files/**/*"
+  # The pre-refactor monolith, kept only for reference.
+  - "exe/pppkg"