fix(ai): Golden Path honors milestone-scoped epics in Current-Focus visibility (computed route stays leaves-only)

Refs #14310 (trigger: the v13.1 docs epic is invisible in the Golden Path despite being v13.1-scoped).

Problem

A milestone-scoped epic is invisible in the entire Golden Path. epic is excluded from both surfaces:

• ai/services/graph/computedGoldenPathRouting.mjs:19 (COMPUTED_RECOMMENDATION_EXCLUDED_LABELS) — the computed route.
• ai/services/graph/issueFocusSections.mjs:19 (CURRENT_FOCUS_EXCLUDED_LABELS) — the Current Release / Incident Focus visibility section (scoreCurrentFocusIssue returns null at line 593 for epic-labeled issues).

So epic #14310 (epic + milestone: v13.1 + OPEN) appears nowhere in sandman_handoff.md, even though it is the v13.1 prio-0 docs initiative. A reader sees its ~23 subs scattered with no grouping signal that they are one initiative.

The split (intended fix — verified architectural call)

• Computed route: keep leaves-only. Routing autonomous pickup (parseGoldenPath) to an epic umbrella is ambiguous (which of 23 subs?). The route must stay actionable leaves — no change to the computedGoldenPathRouting.mjs epic exclusion. (The epic's subs already route once milestone-scoped.)
• Current-Focus visibility: include epics. A focus-signal'd epic (milestone v13.1 / PRIO-ZERO / incident) should surface as an umbrella signal so the handoff shows "epic #N — v13.1 focus, M open subs" instead of orphaned leaves.

Acceptance Criteria

• The Current-Focus candidate builder surfaces epic-labeled issues that carry a focus signal (milestone v13.1 / prio-zero / incident), rendered in the Current Release / Incident Focus section — ideally as an umbrella line with open-sub count, visually distinct from leaf candidates.
• The computed route remains leaves-only (epic still excluded from isActionableComputedRecommendation); add/keep a regression test proving epics never enter the computed route.
• A v13.1-milestoned epic (e.g. #14310) appears in the Current-Focus visibility section after the next golden-path run.
• Unit coverage for both: epic included in focus visibility, epic excluded from the computed route.

Grounding (V-B-A done 2026-06-29)

Verified the two exclusion sites above; confirmed #14310 is epic + milestone: v13.1 + OPEN, so it is currently null from scoreCurrentFocusIssue (line 593) AND non-actionable in the route. Net effect: invisible everywhere.

Authored by Grace (@neo-opus-grace).