feat: add AG-UI channel

[PsiACE]

Summary

• add bub-ag-ui, an AG-UI HTTP/SSE channel for the Bub gateway
• align the integration with Bub 0.4 public contracts and stream event shapes
• use Bub's ag-ui configuration namespace and BUB_AG_UI_* environment variables
• keep Bub as a host-provided dependency instead of declaring it in the package metadata
• add a credential-free end-to-end example with an interactive CopilotKit web frontend
• add package documentation, workspace wiring, tests, and lockfile entries

Migration notes

This ports the AG-UI integration from ob-labs/agentseek at commit a850714a5b2ec1ec18437f7090326b2c47da2495 (Apache-2.0).

The new package is deliberately Bub-native:

• distribution: bub-ag-ui
• Python module: bub_ag_ui
• entry point: ag-ui
• config section: ag-ui
• environment prefix: BUB_AG_UI_
• usage event: bub.usage

No AgentSeek-specific names, aliases, commands, or configuration remain in the package.

End-to-end web example

packages/bub-ag-ui/examples contains:

• a CopilotKit React/Vite chat frontend
• a Copilot Runtime whose AG-UI HttpAgent forwards runs to Bub
• a deterministic streaming Bub model plugin, so the default walkthrough needs no provider credential
• commands that run the complete CopilotChat -> Copilot Runtime -> AG-UI HttpAgent -> bub-ag-ui -> Bub turn path

The documented flow was executed locally through the same /api/copilotkit/agent/default/run route used by the browser. It forwarded to Bub's /agent endpoint and returned RUN_STARTED, state, text, and RUN_FINISHED events. The frontend and gateway then shut down cleanly.

Validation

• uv run --group test pytest -q — 327 passed, 2 skipped
• uv run --group test pytest -q packages/bub-ag-ui/tests — 11 passed
• uv run ruff check packages/bub-ag-ui
• uv run ruff format --check packages/bub-ag-ui
• uv lock --check
• uv build --package bub-ag-ui
• uv build packages/bub-ag-ui/examples/echo-plugin
• frontend npm ci
• frontend npx --yes prettier@3.6.2 --check .
• frontend npm run build (TypeScript and Vite)
• real Copilot Runtime to Bub AG-UI streaming run
• inspected wheel metadata to confirm there is no Requires-Dist: bub