dashboard: merge-gate pills (sentinel, needs-human, unauthorized) show before the PR reaches the gate #14650

Owner's rule (2026-10-27)

"Until the PR is in the merge gate, we should not see things like needs-human or unauthorized or sentinel, because there is nothing we can do about it while PRs are in motion."

Screenshot evidence on the Hive activity monitor: #14399 at CI / Review shows sentinel + needs human pills; #14614 at CI / Review shows sentinel; rationguard#397 at Returned to contributor shows unauthorized.

Problem

stageTrackerModifiers (src/pkg/dashboard/stage_tracker.go) emits every marker it finds — needs-human, needs-decision, unauthorized — regardless of the row's stage, and the sentinel chip (stageTrackerSentinelChipHtml in static/index.html) draws on every lane but 085. All of them also count under Needs attention (stageTrackerNeedsAttention). While the PR is still before the merge gate none of these is something a person can act on, so they are noise that hides the pills that are actionable (hold, draft, stalled).

Proposal

Split the pills into two classes, computed server-side with an explicit actionable_now flag per modifier:

1. Actionable now — always rendered: hold, draft, stalled, regressed (muted stage ↓ unless parked on a person, as today), needs-rebase, blocked, and unauthorized only when the refused step blocks the current stage (unauthorized:approve-workflow-runs — held workflow runs mean CI cannot even start; rationguard#397 is this carve-out, since waiting for the gate would waste the whole CI / review cycle).
2. Merge gate — held back (payload deferred_modifiers) while the PR's stage is before the merge-gate band (stage/080, read from stagelabels.AwaitingMergeAdmission, the same boundary adoption yields at): sentinel (notice and real alert alike), needs-human, needs-decision, and unauthorized for a merge-time step (merge / branch-update 403). Once the PR reaches 080+ they render exactly as today and count toward Needs attention.

The merge-gate cell text ("Sentinel alert · needs human", "Awaiting human: …") is unchanged; issues on their own stages keep every pill; an issue riding its covering PR follows the PR. Labels are untouched — presentation and counting only. Documented as a pill → class → when-shown table in src/docs/dashboard-sections.md.