Pretty-print fleet JSON so the pager can scroll it

[TheGreatAxios]

Summary

Fleet and lifecycle success JSON is pretty-printed with two-space indent so the line pager can scroll it. Error strings stay raw Error: text and are not double-encoded.

JSON.stringify with no indent made a worker report, agent list, or spawn result one enormous line. The TUI pager is line-oriented, so that payload could not be scanned even when it was under the leisure spill budget.

• wait_agents, list_agents, spawn_agent, close_agent, resume_agent, interrupt_agent, and send_input success envelopes go through fleetJson.
• search_agents / read_agent_trace stay multi-line text, not JSON envelopes.
• Tests require a newline and a JSON.stringify(parsed, null, 2) round-trip. This is independent of leisure-on-all-tools.

Verification

• bun run typecheck, bun run build, and bun run test pass
• bun run check passes
• bun test src/subagent/agent-fleet.test.ts src/subagent/lifecycle-tools.test.ts covers pretty-print round-trip and raw Error: prefixes

Fixes CL-7622

[linear-code]

CL-7622