Add task lifecycle telemetry

[stainlu]

this is a small runtime slice from the task telemetry direction in #107.

summary:

• add stable workId/parentWorkId fields and a shared FlueWorkRef type
• emit richer task_start/task_end events with timing, usage, model, role, cwd
• include task telemetry in task tool result details
• roll child task usage into the enclosing prompt/skill response usage
• show task start/done/error lines in flue run

intentionally not doing artifacts/workspace storage here. this just gives task execution a clean event spine that #107/#110 can build on.

verification:

• pnpm --filter @flue/sdk check:types
• pnpm --filter @flue/sdk build
• pnpm --filter @flue/cli build
• pnpm exec biome check packages/sdk/src/types.ts packages/sdk/src/agent.ts packages/sdk/src/session.ts packages/cli/bin/flue.ts

[stainlu]

closing this one after catching up with latest main.

the new run / operation vocabulary in 0.5.x is a better shape than this branch's workId layer, so keeping this open would create parallel lifecycle concepts.

i'll carry any remaining task-specific pieces forward only if they fit cleanly on top of runId / operationId.