feat(pm): give dispatch-gates a third verdict — unreachable (#9939)

* feat(pm): give dispatch-gates a third verdict — unreachable

A gate family whose declared population matches nothing in the tree reports
the same thing as one that scanned a real population and found no problems:
green, silently. One hintCovers sweep over the tracked files names those
families, with the reason each could not reach.

Visibility only: the sweep is cross-cutting, like the unfiltered-workflow
count beside it, so no existing verdict moves and the derived gate set is
unchanged.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01XqDQYVU5smx29ts9pAErja

* docs(pm): state the third verdict in the dispatch-gates header contract

The header lists what each verdict means and what the exit code promises;
leaving the sweep out of it would be the same drift this file polices.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01XqDQYVU5smx29ts9pAErja

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Co-authored-by: Claude <noreply@anthropic.com>

Author: os-steve