feat: stream provider output to compatible channels

[alanbulan]

Summary

• prefer StreamingProvider.ChatStream in the agent loop when the active channel supports incremental updates
• add Pico channel streaming primitives plus message.delete handling so the web UI can render partial output safely
• add focused regression tests for OpenAI-compatible stream deltas, Pico channel streaming, and agent-loop streaming dispatch

Why

Some OpenAI-compatible upstreams return choices[0].message.content = null for non-streaming chat completions while producing valid SSE text when stream=true. This change lets PicoClaw use the existing streaming-capable provider path and channel abstractions to surface partial output in the UI and avoid empty final responses on compatible channels.

Test plan

• go test ./pkg/channels/pico -run 'Test(DeleteMessage_SendsDeleteEvent|BeginStream_SendsCreateAndUpdatesMessage)' -v
• go test ./pkg/providers/openai_compat -run TestProviderChatStream_ParsesTextDeltas -v
• go test ./pkg/agent -run TestProcessMessage_StreamsToChannelWhenProviderSupportsStreaming -v
• make check (not run locally in this environment)
• frontend typecheck / lint for web/frontend (dependencies not installed in this environment)

AI usage disclosure

This PR was prepared with AI assistance for code analysis, implementation drafting, and test scaffolding; all changes were reviewed and validated with targeted local tests before submission.

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[alanbulan]

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[sipeed-bot]

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