fix(session): persist the delegation depth in the session header

A subagent child's recursion depth lived only in runtime AgentOptions,
so a persisted child came back from resume counted as top-level and
maxDepth stopped binding after every restart. Add
SessionHeader.delegationDepth, round-trip it through the JSONL and
SQLite backends (SQLite schema v5), restore it on agent-loop resume,
and write it when the in-process backends create a child. The seam now
owns the shared depth vocabulary (delegationDepthOf): the persisted
header is authoritative and monotone — runtime options may deepen it
but never lower it.

packages/core/agent-loop/src/index.ts

@@ -625,6 +625,7 @@ export class AgentLoop extends Service implements AgentFactory {
           ...loaded.meta.cwd === undefined ? {} : { cwd: loaded.meta.cwd },
           ...loaded.meta.parentSession === undefined ? {} : { parentSession: loaded.meta.parentSession },
           ...loaded.meta.seedLength === undefined ? {} : { seedLength: loaded.meta.seedLength },
+          ...loaded.meta.delegationDepth === undefined ? {} : { delegationDepth: loaded.meta.delegationDepth },
         },
       })
       const agent = transaction.prepare(agentOptions, session, this.maxParallelToolCalls)