Fix AIModel ALL_MODELS class-init cycle (NPE in CognitiveRelayCostTest et al)

[wow-miley]

Summary

• Declares ALL_MODELS as by lazy { listOf(...) } in the companion objects of AIModel_Claude, AIModel_Gemini, and AIModel_OpenAI to break a JVM class-initialization cycle.
• Without lazy, if any data object (e.g. AIModel_Gemini.Flash_2_5) was accessed before its companion finished initializing, the companion's <clinit> would observe that object's INSTANCE as null and permanently commit a poisoned list — causing CognitiveRelayCostTest, RouteCostReportTest, and ModelDescriptorTest to NPE when run as part of the full suite but pass in isolation.
• Updates the defaultModelDescriptors() kdoc in ModelDescriptorRegistry to explain the actual fix.

Why it passed CI

The failures are ordering-dependent: tests pass when run with --tests filter (fresh JVM, no prior class contamination) but fail in the full jvmTest run when another test class first touches a model data object directly and poisons ALL_MODELS. If CI was hitting cached Gradle test results from before the new test files were introduced, the failures never surfaced.

Test plan

• ./gradlew :ampere-core:jvmTest passes (all 1366 tests, 0 failures)
• ./gradlew ktlintFormat clean

🤖 Generated with Claude Code

[github-actions]

Concept staleness check — 1 warning(s).

These changes touched files listed under a concept's tracked_sources without updating the concept file. Either update the concept (and bump last_verified) or include a Concept-Verified: <ConceptName> trailer in a commit on this PR.

Validator output

⚠ Concept 'CognitiveRelay' may be stale: ampere-core/src/commonMain/kotlin/link/socket/ampere/agents/domain/routing/capability/ModelDescriptorRegistry.kt matches tracked_sources pattern ampere-core/src/commonMain/kotlin/link/socket/ampere/agents/domain/routing/** but docs/concepts/cognitive-relay.md was not modified.

validate-concepts: 1 warning(s). Validator informs; never blocks.