fix(ui-phase): suggest discuss-phase when CONTEXT.md is missing (#1952) (#1964)

The Next Up block always suggested /gsd-plan-phase, but plan-phase
redirects to discuss-phase when CONTEXT.md doesn't exist. This caused
a confusing two-step redirect ~90% of the time since ui-phase doesn't
create CONTEXT.md.

Conditionally suggest discuss-phase or plan-phase based on CONTEXT.md
existence, matching the logic in progress.md Route B.

Co-authored-by: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

get-shit-done/workflows/ui-phase.md

@@ -261,11 +261,17 @@ Dimensions: 6/6 passed
 
 ## ▶ Next Up
 
+{If CONTEXT.md exists for this phase:}
 **Plan Phase {N}** — planner will use UI-SPEC.md as design context
 
-`/clear` then:
+`/clear` then: `/gsd-plan-phase {N}`
 
-`/gsd-plan-phase {N}`
+{If CONTEXT.md does NOT exist:}
+**Discuss Phase {N}** — gather implementation context before planning
+
+`/clear` then: `/gsd-discuss-phase {N}`
+
+(or `/gsd-plan-phase {N}` to skip discussion)
 
 ───────────────────────────────────────────────────────────────
 ```