Merge native Claude Code integration with explicit opt-in

# Conflicts:
#	src/agent/teacher-agent.ts
#	src/server/keys.ts

src/core/contracts/claude-code.ts

@@ -0,0 +1,12 @@
+export const CLAUDE_CODE_PROVIDER = "claude-code";
+
+/** Installation/version only. Never an authentication, quota, or billing verdict. */
+export interface ClaudeCodeStatus {
+	state: "not-found" | "meets-minimum" | "update-required" | "version-unchecked";
+	version: string | null;
+	minimumVersion: string;
+}
+
+// --permission-prompts none was added in 2.1.259. This is not an official
+// minimum-version/stability contract for claudecode/toolUseId; admission fails closed.
+export const CLAUDE_CODE_MINIMUM_VERSION = "2.1.259";

src/core/contracts/models.ts

@@ -21,9 +21,11 @@ export interface ModelCatalogEntry {
 	 * thinkingLevelMap — e.g. GLM-5.3-flash: low|high|max). The thinking
 	 * picker lists exactly these; anything else is a lie. */
 	supportedLevels: ThinkingLevel[];
-	cost: { input: number; output: number; cacheRead: number; cacheWrite: number };
-	contextWindow: number;
-	maxTokens: number;
+	/** Null when LearnHarness cannot estimate prices for this connection. */
+	cost: { input: number; output: number; cacheRead: number; cacheWrite: number } | null;
+	/** Null for native CLI aliases whose effective limits are session-specific. */
+	contextWindow: number | null;
+	maxTokens: number | null;
 }
 
 /** The conversation agents' resolved default (BYOK, owner 2026-09-22): the