docs(testing): add PlanAction JWT hydration implementation guide

Add comprehensive implementation guide for creating PlanAction claims
via hydratePlan() function pattern, following established hydrateGive()
and hydrateOffer() patterns.

Changes:
- Add hydrate-plan-implementation-guide.md with complete implementation
  details, usage examples, and testing recommendations
- Link implementation guide from getting-valid-plan-ids.md Method 6
- Link implementation guide from localhost-testing-guide.md Option B

The guide provides:
- Complete hydratePlan() function implementation
- PlanActionClaim interface structure
- Helper functions (createAndSubmitPlan, editAndSubmitPlan)
- Usage examples and edge cases
- Testing recommendations and security considerations

This complements plan creation documentation by showing how to
programmatically construct valid PlanAction JWTs for POST /api/v2/claim.

docs/localhost-testing-guide.md

@@ -70,6 +70,8 @@ If you have your own localhost API server, you must create plans first. Plans ar
 
 **Note**: Creating PlanAction JWTs requires DID signing and proper claim structure, which is complex. For quick testing, **Option A (test API server) is recommended**.
 
+**Implementation Reference**: If you need to programmatically create PlanAction JWTs, see [`docs/hydrate-plan-implementation-guide.md`](./hydrate-plan-implementation-guide.md) for the complete implementation pattern, including `hydratePlan()` function and helper utilities.
+
 Then verify your setup:
 
 ```bash