feat: add agenticloop watch TUI and SSE events

[mattjoyce]

Summary

• add authenticated SSE endpoint GET /v1/runs/{run_id}/events
• stream initial snapshot, then run.updated, step.created, step.updated, and terminal stream.closed
• add agenticloop watch Bubble Tea TUI client for run event streaming
• style the watch UI with a blue/orange theme and event timeline
• update usage/docs in README.md
• add API signature tests for stable change detection (internal/api/handlers_stream_test.go)

Testing

• go test ./internal/api ./internal/store

Notes

• Full go test ./... and go test ./cmd/agenticloop are currently blocked by an existing upstream dependency issue in github.com/bytedance/sonic (undefined: GoMapIterator) on this environment.