fix: manager workflow delegates to Skill pipeline instead of raw Task prompts

Background agents for plan/execute now call Skill(gsd:plan-phase) and
Skill(gsd:execute-phase) instead of reimplementing the workflow steps
inline. This ensures local patches, quality gates, and proper branching
are respected. Also removes --no-verify anti-pattern and specifies
exact Skill names in error handler fallbacks.

Fixes #1453

commands/gsd/manager.md

@@ -8,6 +8,7 @@ allowed-tools:
   - Glob
   - Grep
   - AskUserQuestion
+  - Skill
   - Task
 ---
 <objective>