[Pi 8/8] Activate Pi in supported inventory and release cohort

## Summary

Activate Pi as a supported open-source NemoClaw agent runtime only after the product decision, implementation, policy, qualification, and documentation gates are complete.

Parent epic: #7923

Depends on #7926, #7925, #7927, #7930, #7924, #7928, and #7929.

Target completion: 2026-08-25.

## Open-source scope

This issue governs the public NemoClaw product surface and release artifacts. It does not require approval from an NVIDIA-internal deployment program. Organizations may apply separate policies when deploying the open-source project in their own environments.

## Problem Statement

Candidate Pi artifacts can be built and qualified before Pi is publicly selectable. Activating Pi earlier would expose an incomplete runtime or publish a cohort without complete exact-candidate evidence.

No existing Pi subissue owns the final atomic change that enables selection, updates the supported inventory, publishes the complete image cohort, and verifies the released product surface.

## Desired Behavior

After every prerequisite is complete, one final activation change exposes Pi through interactive onboarding and `nemoclaw onboard --agent pi`, adds Pi to the supported agent inventory and atomic managed-image cohort, and binds the release to exact qualification evidence.

Activation is atomic: a release must not advertise Pi unless its package, image, startup profile, inference route, policy, lifecycle, documentation, and evidence identities agree.

## Scope

- Verify every Pi prerequisite and acceptance receipt.
- Add Pi to the supported and selectable agent inventory.
- Enable interactive and non-interactive onboarding selection.
- Add Pi to the atomic managed-image publication cohort for each accepted platform and architecture.
- Verify catalog, manifest, startup-profile, policy, and image-digest identity agreement.
- Run the release-blocking exact-candidate qualification gate.
- Publish the supported compatibility matrix and known limitations.
- Confirm rollback behavior if activation or publication fails.

## Constraints and Non-goals

- Do not activate Pi while any prerequisite remains incomplete.
- Do not publish a partial platform or architecture cohort.
- Do not use mutable image tags, unpinned packages, or manual sandbox edits.
- Do not add a Pi-specific Docker, Podman, MXC, lifecycle, or recovery branch.
- Do not claim support for a compute runtime, provider, API family, or platform outside the accepted matrix. Do not accept a model that fails the accepted validation rules.
- Do not make NVIDIA-internal deployment approval a prerequisite for the open-source release.

## Acceptance Criteria

- [ ] #7926, #7925, #7927, #7930, #7924, #7928, and #7929 are complete.
- [ ] Pi is present in the supported agent inventory with canonical ID `pi`.
- [ ] Interactive onboarding can select Pi.
- [ ] `nemoclaw onboard --agent pi` selects and persists Pi.
- [ ] The atomic managed-image cohort contains every accepted Pi platform and architecture digest.
- [ ] Catalog, manifest, startup profile, policy, runtime package, and managed-image identities match the exact-candidate evidence.
- [ ] Release-blocking qualification passes from the exact candidate commit without manual repair.
- [ ] Unsupported platforms, compute runtimes, providers, and API families remain unavailable with actionable errors. Models that fail the accepted validation rules are rejected.
- [ ] Documentation and release notes describe the exact supported matrix and limitations.
- [ ] A failed activation or publication does not leave Pi partially advertised or partially published.

## Category

Feature

## Checklist

- [x] I searched existing issues and this is not a duplicate.
- [x] I described the problem and desired behavior.

Start with the prerequisite issues #7926, #7925, #7927, #7930, #7924, #7928, and #7929, then inspect the supported agent inventory and the `nemoclaw onboard --agent pi` entry point. Done means Pi is atomically selectable and published only for the accepted platform and architecture cohort, exact-candidate qualification passes, unsupported options fail clearly, and the compatibility matrix and limitations are documented.

Written by the indexing model from the issue text.