fix(web): deduplicate subagent navigation

packages/core/session/src/index.ts

@@ -138,6 +138,9 @@ function snapshotSessionHeader(id: SessionId, source?: SessionHeader): SessionHe
     && (typeof record.seedLength !== 'number' || !Number.isSafeInteger(record.seedLength) || record.seedLength < 0)) {
     throw new Error('session header seedLength must be a non-negative safe integer')
   }
+  if (record.origin !== undefined && record.origin !== 'subagent') {
+    throw new Error('session header origin must be "subagent"')
+  }
   if (record.delegationDepth !== undefined
     && (typeof record.delegationDepth !== 'number' || !Number.isSafeInteger(record.delegationDepth) || record.delegationDepth < 0)) {
     throw new Error('session header delegationDepth must be a non-negative safe integer')
@@ -837,6 +840,7 @@ export class SessionStore extends Service {
       ...meta?.cwd === undefined ? {} : { cwd: meta.cwd },
       ...meta?.parentSession === undefined ? {} : { parentSession: meta.parentSession },
       ...meta?.seedLength === undefined ? {} : { seedLength: meta.seedLength },
+      ...meta?.origin === undefined ? {} : { origin: meta.origin },
       ...meta?.delegationDepth === undefined ? {} : { delegationDepth: meta.delegationDepth },
     }
     return new Session(sessionId, seed, header)

packages/core/session/src/types.ts

@@ -59,6 +59,11 @@ export interface SessionHeader {
    * boundary lets resume and replay distinguish parent history from child work.
    */
   readonly seedLength?: number
+  /**
+   * Coarse product classification for a session created as a subagent child.
+   * This is presentation metadata, not proof that the child is continuable.
+   */
+  readonly origin?: 'subagent'
   /**
    * Delegation depth: absent (zero) for a top-level session, parent depth + 1
    * for a subagent child. Persisted so a recursion budget survives restart and
@@ -84,6 +89,7 @@ export interface CreateSessionOptions {
     readonly parentSession?: SessionId
     readonly createdAt?: number
     readonly seedLength?: number
+    readonly origin?: 'subagent'
     readonly delegationDepth?: number
   }
 }