Extract the shared Golden-Path capture-timestamp formatter out of GoldenPathSynthesizer

Refs #14281

Extract the shared Golden-Path capture-timestamp formatter out of GoldenPathSynthesizer — a Phase-A shared-helper prerequisite of the SRP-decomposition (#14281).

Context (the decomposition is dependency-ordered)

V-B-A on the remaining GoldenPathSynthesizer lanes revealed they are coupled via shared static helpers — a lane cluster cannot be cleanly extracted as one PR while it depends on a shared static still living in GoldenPathSynthesizer, and extracting a lane's predicates-without-render would be a decider/wiring micro-slice. So the decomposition must home the shared helpers FIRST.

formatGoldenPathCapturedAt is one such shared helper: a pure capture-timestamp formatter used by 4 render sites (renderComputedGoldenPathContradictionSection L427, plus L492/L554, and the handoff render at L1158). It blocks the clean extraction of lane 5 (computed-recommendation, whose render uses it).

The Fix

• Move formatGoldenPathCapturedAt to a focused pure module ai/services/graph/goldenPathTimestamp.mjs.
• GoldenPathSynthesizer imports it back + re-exports it (public API stable); the 4 call-sites resolve to the imported function.
• Add focused unit tests (valid date, string date, non-finite → 'unknown').

Acceptance Criteria

• formatGoldenPathCapturedAt lives in ai/services/graph/goldenPathTimestamp.mjs; GoldenPathSynthesizer imports + re-exports it.
• All 4 GPS call-sites resolve to the imported function; behavior-preserving.
• Focused unit tests; node --check clean; the existing GoldenPathSynthesizer spec stays green.

Out of Scope

• The lane clusters that this unblocks (lane 5 etc.) — separate leaves once Phase-A helpers are homed.
• Any behavior change (pure move + re-export).

Origin Session ID: 090a68e6-1a28-4b20-a5fd-842ebac3e729. Authored by Grace (@neo-opus-grace, Claude Opus 4.8, Claude Code).