[External State] squad cast does not follow externalized state for agent discovery #1399

Summary

squad cast does not follow externalized state markers for project agent discovery.

Evidence

• packages/squad-cli/src/cli/commands/cast.ts:22,31-36
  • uses resolveSquadPaths(cwd), then LocalAgentSource(agentBase)
• packages/squad-sdk/src/config/agent-source.ts:36,111-115
  • LocalAgentSource resolves from <basePath>/.squad/agents
• packages/squad-sdk/src/resolution.ts:368-372
  • resolveSquadPaths resolves teamDir from teamRoot (not external stateLocation/projectKey)
• packages/squad-cli/src/cli/commands/externalize.ts:101-103
  • externalize sets teamRoot: '.' + stateLocation: 'external'

Expected

Agent discovery for cast should resolve through effective state location when externalized.

Actual

Discovery path is derived from team-root semantics and can remain repo-local.