fix: extract_facts fallback for facts-prefixed selectors in datalog verification

Root cause: ActionEnvelope.to_yaml_value() creates a Mapping from the
facts HashMap without a 'facts:' wrapper key, but rulespec selectors
may include a 'facts.' prefix (e.g. 'facts.feature.done' instead of
'feature.done'). This caused zero facts to be extracted, making all
predicate evaluations fail.

Fix: extract_facts() now tries the selector against the unwrapped
envelope value first, and if empty, retries against a facts-wrapped
version as fallback.

Also:
- Strengthened write_envelope tool description to require top-level
  facts: key, file paths for evidence, and allow free-form notes
- Updated system prompt with matching rules
- Added 6 new tests (4 unit, 2 integration)
- Strengthened existing integration test to verify fact count > 0

prompts/system/native.md

@@ -93,6 +93,9 @@ facts:
 ```
 
 **Rules:**
+- All fact groups MUST go under the top-level `facts:` key. No other top-level keys except envelope metadata (e.g. `type:`)
+- Use file paths as evidence values so the validator can check them: `"src/foo.rs"`, `"src/foo.rs:42"`, `"tests/bar.rs::test_name"`
+- Free-form notes are allowed alongside file paths (e.g. `notes: "Refactored from old module"`)
 - Selectors in `analysis/rulespec.yaml` (e.g., `csv_importer.capabilities`) are evaluated against envelope facts
 - Use dot notation for nested access: `api_changes.breaking`
 - Use `null` to explicitly assert absence (for `not_exists` predicates)