refactor(core): simplify system context to a belief model

Reconcile never rewrites the baseline; only rebaseline (compaction) and
initialize (first turn) produce baseline text. The durable Applied record
tracks what the model was last told, per source.

- Incompatible stored values re-announce the source baseline as an update
  instead of forcing a full replacement
- Sources removed without removal text retain the model's belief silently;
  entries self-clean at the next rebaseline
- Rebaseline restates unobservable sources from their last-applied values
  instead of blocking; ReplacementBlocked and ReplacementReady are gone
- Rename Snapshot to Applied and Generation to Baseline {text, applied}

packages/core/test/reference-guidance.test.ts

@@ -16,10 +16,10 @@ describe("ReferenceGuidance", () => {
       const guidance = yield* ReferenceGuidance.Service
       const generation = yield* SystemContext.initialize(yield* guidance.load())
 
-      expect(generation.baseline).toContain("<available_references>")
-      expect(generation.baseline).toContain("<name>docs</name>")
-      expect(generation.baseline).toContain("<path>/docs</path>")
-      expect(generation.baseline).toContain("<description>Use for product documentation</description>")
+      expect(generation.text).toContain("<available_references>")
+      expect(generation.text).toContain("<name>docs</name>")
+      expect(generation.text).toContain("<path>/docs</path>")
+      expect(generation.text).toContain("<description>Use for product documentation</description>")
     }).pipe(
       Effect.provide(
         guidanceLayer(
@@ -47,7 +47,7 @@ describe("ReferenceGuidance", () => {
     Effect.gen(function* () {
       const guidance = yield* ReferenceGuidance.Service
       const generation = yield* SystemContext.initialize(yield* guidance.load())
-      expect(generation.baseline).toBe("")
+      expect(generation.text).toBe("")
     }).pipe(Effect.provide(guidanceLayer(Layer.mock(Reference.Service, { list: () => Effect.succeed([]) })))),
   )
 
@@ -55,7 +55,7 @@ describe("ReferenceGuidance", () => {
     Effect.gen(function* () {
       const guidance = yield* ReferenceGuidance.Service
       const generation = yield* SystemContext.initialize(yield* guidance.load())
-      expect(generation.baseline).toBe("")
+      expect(generation.text).toBe("")
     }).pipe(
       Effect.provide(
         guidanceLayer(