#1129 feat(spawn): add task-exit mode

Summary

• Add optional task-exit spawn mode to broker HTTP spawn handling and harness-driver spawn inputs.
• Add agent-relay local agent spawn/new --spawn-mode task-exit and --exit-after-task flags.
• Extend MCP add_agent with task-exit options by appending the post-task /exit instruction.
• Document the user-facing change in CHANGELOG.md.

Verification

• cargo test -p agent-relay-broker exit_after_task_instruction_appends_clean_exit_contract
• cargo test -p agent-relay-broker spawn_route_forwards_extended_fields
• npx vitest run packages/harness-driver/src/agent-result.test.ts packages/cli/src/cli/commands/local-agent.test.ts packages/cli/src/cli/agent-relay-mcp.startup.test.ts
• npm run build:harness-driver
• npm --prefix packages/cli run build

No merge performed.