chore(repo): migrate github label taxonomy - #454

PR NVIDIA#2524 replaced the `state:*` lifecycle vocabulary with an `agent:*`
namespace whose labels were never created on GitHub. `gh label list`
returns nine `state:*` labels and zero `agent:*` labels, so every gate
referring to the `agent:*` names was unreachable.

This had a live functional consequence beyond documentation drift.
`.github/workflows/stale.yml` exempted five phantom labels and omitted
the four real in-flight states. As of 2026-09-09, 18 open issues carry
`state:stale` while sitting in a state the workflow intends to exempt,
including issues with open PRs and issues mid-implementation. That count
was 17 on 2026-09-02, so the exposure is still growing.

Restore the real vocabulary across the skills, contributor docs, and
the stale workflow, and add `.agents/issue-lifecycle.yaml` as the
canonical machine-readable source of truth. `agent:plan-requested` has
no `state:*` predecessor; queue-mode planning is authorized by
`state:accepted` or roadmap placement, which is what that label already
means. The file records the phantom names under `never_existed` so
future readers recognize stale references instead of treating them as
valid gates.

Document staleness as an orthogonal boundary rather than a lifecycle
state, complete the CONTRIBUTING.md state table from four states to
eight, and add a two-way drift check to `sync-agent-infra` so
documentation and GitHub cannot diverge again unnoticed.

PR NVIDIA#2809 fixed `.agents/skills/triage-issue/SKILL.md` alone and was
closed unmerged on 2026-09-08. This change absorbs that file so the
reconciliation stays complete.

Refs: NVIDIA#2524, NVIDIA#2809, NVIDIA#454
Signed-off-by: Jim Meyer <[email protected]>