docs: document issue lifecycle labels and roadmap sequencing

[krishicks]

Summary

Add a published Issue Triage and Lifecycle page and align AGENTS.md, CONTRIBUTING.md, README.md, the PR template, and the issue-handling skills on the state:/agent: label model.

This is meant to make it easier for us to manage the open issues and PRs in the project. Presently it's not easy to know which issues are validated and/or accepted, for example. This also clarifies the state machine for issues that are going to be owned by an agent.

Related Issue

Changes

Testing

• mise run pre-commit passes
• Unit tests added/updated
• E2E tests added/updated (if applicable)

Checklist

• Follows Conventional Commits
• Commits are signed off (DCO)
• Architecture docs updated (if applicable)

[github-actions]

🌿 Preview your docs:https://nvidia-preview-pr-2524.docs.buildwithfern.com/openshell

[pimlock]

This looks good IMO, I haven't reviewed all the small changes, my assumption is the overall approach was to define the new process (the issue-lifecycle doc), give that to the agent to update all existing skills/documentation around this?

Once this lands, we'll need to migrate some of the existing labels, right? E.g. the state:... -> agent:....

[matthewgrossman]

overall lgtm, also feels reasonable to try out and adjust the process as we find parts we like/don't like (like we're doing now!)