feat: add experimental Pi SDK provider

[Artur Leao (arturleao)]

Summary

• Add an experimental pi provider backed by a one-request Node runner.
• Package Pi runner and locked Node manifest with Conductor while excluding node_modules.
• Add provider registration, diagnostics, capability declaration, example workflow, and regression coverage.
• Reap bridge processes after terminal messages so completed Pi agents cannot hang waiting for Node session teardown.

Notes

• Requires Node.js 22.19.0+ and npm ci in src/conductor/providers/_pi/.
• Pi model authentication and configuration remain managed by Pi.
• Workflow MCP servers and Conductor per-agent tool allowlists are intentionally unsupported by this provider.

Testing

• tests/test_providers/test_pi.py (9 passed)
• Pi two-agent end-to-end smoke workflow against a real authenticated Pi model
• Node runner syntax check, scoped Ruff, and scoped type checks
• Built wheel/sdist verification: runner and manifests included; node_modules excluded

Full-suite failures outside this change were observed in existing ACA/Fleet/MCP/performance suites; this PR does not claim a full green suite.

[Artur Leao (arturleao)]

Artur Leao (Artur Leao (@arturleao)) please read the following Contributor License Agreement(CLA). If you agree with the CLA, please reply with the following information.

@microsoft-github-policy-service agree [company="{your company}"]

Options:

• (default - no company specified) I have sole ownership of intellectual property rights to my Submissions and I am not making Submissions in the course of work for my employer.

@microsoft-github-policy-service agree

• (when company given) I am making Submissions in the course of work for my employer (or my employer has intellectual property rights in my Submissions by contract or applicable law). I have permission from my employer to make Submissions and enter into this Agreement on behalf of my employer. By signing below, the defined term “You” includes me and my employer.

@microsoft-github-policy-service agree company="Microsoft"

Contributor License Agreement

@microsoft-github-policy-service agree