fix(workflows): standardize agent spawn types across all workflows

Replace general-purpose workaround pattern with named subagent types:
- plan-phase: researcher and planner now spawn as gsd-phase-researcher/gsd-planner
- new-project: 4 researcher spawns now use gsd-project-researcher
- research-phase: researcher spawns now use gsd-phase-researcher
- quick: planner revision now uses gsd-planner
- diagnose-issues: debug agents now use gsd-debugger (matches template spec)

Removes 'First, read agent .md file' prompt prefix — named agent types
auto-load their .md file as system prompt, making the workaround redundant.

Preserves intentional general-purpose orchestrator spawns in discuss-phase
and plan-phase (auto-advance) where the agent runs an entire workflow.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

commands/gsd/research-phase.md

@@ -135,8 +135,8 @@ Write to: .planning/phases/${PHASE}-{slug}/${PHASE}-RESEARCH.md
 
 ```
 Task(
-  prompt="First, read ~/.claude/agents/gsd-phase-researcher.md for your role and instructions.\n\n" + filled_prompt,
-  subagent_type="general-purpose",
+  prompt=filled_prompt,
+  subagent_type="gsd-phase-researcher",
   model="{researcher_model}",
   description="Research Phase {phase}"
 )
@@ -171,8 +171,8 @@ Continue research for Phase {phase_number}: {phase_name}
 
 ```
 Task(
-  prompt="First, read ~/.claude/agents/gsd-phase-researcher.md for your role and instructions.\n\n" + continuation_prompt,
-  subagent_type="general-purpose",
+  prompt=continuation_prompt,
+  subagent_type="gsd-phase-researcher",
   model="{researcher_model}",
   description="Continue research Phase {phase}"
 )