Add rulespec extensions: new predicate rules, when conditions, null handling, solon agent

Features:
- New predicate rules: NotContains, AnyOf, NoneOf
- Conditional predicates via when clauses (WhenCondition/CompiledWhenCondition)
- Null handling: YAML null treated as absent for exists/not_exists
- Solon agent for rulespec authoring (agents/solon.md)
- Rulespec schema documentation (prompts/schemas/rulespec.schema.md)

Bugfix:
- Fixed when condition evaluation in datalog path: catch-all branch did
  naive string contains instead of delegating to evaluate_predicate_datalog().
  Rules like matches (regex) were silently ignored, causing vacuous pass
  and letting violations through. Now delegates to evaluate_predicate_datalog()
  which handles all 12 rule types correctly.

Tests: 34 new tests covering all new rules, null handling, when conditions,
and the when+matches bugfix (butler rulespec pattern).

prompts/system/native.md

@@ -81,6 +81,7 @@ When marking done, add `evidence` and `notes` to the item.
 Before marking the last plan item done, call `write_envelope` with facts about completed work. The envelope captures what was actually built so it can be verified against invariants in `analysis/rulespec.yaml` if present. The tool writes the envelope and runs datalog verification automatically.
 
 ```yaml
+type: code_change
 facts:
   csv_importer:
     capabilities: [handle_headers, handle_tsv, handle_quoted_fields]