docs(refactor): add integration point refactor analysis and implementation plan

Add comprehensive documentation and implementation artifacts for refactoring
the plugin to use app-provided content fetchers instead of hardcoded TimeSafari
integration.

Changes:
- Add integration-point-refactor-analysis.md with complete ADR, interfaces,
  migration plan, and 7-PR breakdown
- Add INTEGRATION_REFACTOR_QUICK_START.md for quick reference on new machines
- Add src/types/content-fetcher.ts with TypeScript SPI interfaces
- Add examples/native-fetcher-android.kt with Kotlin implementation skeleton
- Add examples/js-fetcher-typescript.ts with TypeScript implementation skeleton
- Add tests/fixtures/test-contract.json for golden contract testing

Architecture Decisions:
- Dual-path SPI: Native Fetcher (background) + JS Fetcher (foreground only)
- Background reliability: Native SPI only, no JS bridging in workers
- Reversibility: Legacy code behind feature flag for one minor release
- Test contract: Single JSON fixture for both fetcher paths

This provides complete specification for implementing the refactor in 7 PRs,
starting with SPI shell and progressing through background workers, deduplication,
failure policies, and finally legacy code removal.

All documentation is self-contained and ready for implementation on any machine.

tests/fixtures/test-contract.json

@@ -0,0 +1,70 @@
+{
+  "description": "Golden contract test fixture - used by both JS and Native fetcher tests",
+  "version": "1.0.0",
+  "context": {
+    "trigger": "scheduled",
+    "scheduledTime": 1730179200000,
+    "fetchTime": 1730175600000,
+    "metadata": {
+      "testRun": true,
+      "runId": "test-run-001"
+    }
+  },
+  "expectedOutput": [
+    {
+      "id": "offer_123",
+      "title": "New Offer: Community Garden Project",
+      "body": "A new volunteer opportunity is available",
+      "scheduledTime": 1730179200000,
+      "fetchTime": 1730175600000,
+      "mediaUrl": "https://example.com/images/garden.jpg",
+      "ttlSeconds": 86400,
+      "dedupeKey": "offer_123_2025-10-29T12:00:00Z",
+      "priority": "default",
+      "metadata": {
+        "offerId": "123",
+        "issuerDid": "did:key:test123",
+        "source": "offers_to_person"
+      }
+    },
+    {
+      "id": "project_456",
+      "title": "Community Center Project Updated",
+      "body": "New updates for Community Center Project",
+      "scheduledTime": 1730179200000,
+      "fetchTime": 1730175600000,
+      "mediaUrl": null,
+      "ttlSeconds": 86400,
+      "dedupeKey": "project_https://endorser.ch/entity/01ABC_789xyz",
+      "priority": "default",
+      "metadata": {
+        "planHandleId": "https://endorser.ch/entity/01ABC",
+        "jwtId": "789xyz",
+        "source": "project_updates"
+      }
+    }
+  ],
+  "failureScenarios": [
+    {
+      "name": "network_timeout",
+      "simulate": "timeout_5000ms",
+      "expectedBehavior": "retry_with_backoff"
+    },
+    {
+      "name": "auth_401",
+      "simulate": "http_401",
+      "expectedBehavior": "unretryable_error_logged"
+    },
+    {
+      "name": "rate_limit_429",
+      "simulate": "http_429_retry_after_60",
+      "expectedBehavior": "honor_retry_after_header"
+    },
+    {
+      "name": "partial_failure",
+      "simulate": "malformed_item_in_array",
+      "expectedBehavior": "skip_bad_item_enqueue_valid"
+    }
+  ]
+}
+