feat: context window from model endpoints + percentage-based auto-compact

ModelInfo (kernel contract):
- Add contextWindow?: number field

OpenAI-stream listModels:
- Parse contextWindow from common field names (context_length,
  context_window, max_context_length, max_tokens)

Transport-contract:
- ModelsResponse: add optional modelInfo map (model name → { contextWindow? })
- Add ModelMetadata type
- Rename CompactThresholdResponse → CompactPercentResponse
- Rename SetCompactThresholdRequest → SetCompactPercentRequest

Credential store:
- Add getModelInfo(modelName) method — resolves full ModelInfo
  (including contextWindow) for a <credential>/<model> string

Transport-http:
- GET /models now includes modelInfo with contextWindow per model
- Rename compact-threshold endpoints → compact-percent

Session-orchestrator:
- Auto-compact now uses contextSize (not overcounted usage.inputTokens)
  compared against contextWindow * (percent / 100)
- Default percent: 85 (was flat 350000)
- resolveModelInfo dep added to look up contextWindow
- Passes modelName from the settled turn to the compaction service

Conversation store:
- Rename getCompactThreshold/setCompactThreshold → getCompactPercent/setCompactPercent
- compactThresholdKey → compact-percent key

1 files changed, 25 insertions, 1 deletions

diff --git a/packages/credential-store/src/registry.ts b/packages/credential-store/src/registry.ts
index c1ba771..89d8084 100644
--- a/packages/credential-store/src/registry.ts
+++ b/packages/credential-store/src/registry.ts
@@ -1,4 +1,4 @@
-import type { ProviderContract } from "@dispatch/kernel";
+import type { ModelInfo, ProviderContract } from "@dispatch/kernel";
 
 export interface Credential {
 	readonly name: string;
@@ -24,6 +24,13 @@ export interface CredentialStore {
 	 * missing or has no listModels.
 	 */
 	listCatalog(): Promise<readonly string[]>;
+
+	/**
+	 * Returns the full `ModelInfo` for a `<credentialName>/<model>` string, or
+	 * undefined if unknown. Caches the result of `listModels` per credential.
+	 * Used to look up `contextWindow` for auto-compaction.
+	 */
+	getModelInfo(modelName: string): Promise<ModelInfo | undefined>;
 }
 
 export interface CredentialStoreDeps {
@@ -76,5 +83,22 @@ export function createCredentialStore(deps: CredentialStoreDeps): CredentialStor
 
 			return results;
 		},
+
+		async getModelInfo(modelName: string): Promise<ModelInfo | undefined> {
+			const slashIndex = modelName.indexOf("/");
+			if (slashIndex === -1) return undefined;
+			const credentialName = modelName.slice(0, slashIndex);
+			const modelId = modelName.slice(slashIndex + 1);
+			if (!modelId) return undefined;
+
+			const providerId = credentialMap.get(credentialName);
+			if (providerId === undefined) return undefined;
+
+			const provider = deps.getProvider(providerId);
+			if (provider?.listModels === undefined) return undefined;
+
+			const models = await provider.listModels();
+			return models.find((m) => m.id === modelId);
+		},
 	};
 }