fix: add <available_agent_types> to all workflows spawning named agents

PR #1139 added <available_agent_types> sections to execute-phase.md and
plan-phase.md to prevent /clear from causing silent fallback to
general-purpose. However, 14 other workflows and 2 commands that also
spawn named GSD agents were missed, leaving them vulnerable to the same
regression after /clear.

Added <available_agent_types> listing to: research-phase, quick,
audit-milestone, diagnose-issues, discuss-phase-assumptions,
execute-plan, map-codebase, new-milestone, new-project, ui-phase,
ui-review, validate-phase, verify-work (workflows) and debug,
research-phase (commands).

Added regression test that enforces every workflow/command spawning
named subagent_type must have a matching <available_agent_types>
section listing all spawned types.

Fixes #1357

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

commands/gsd/debug.md

@@ -17,6 +17,11 @@ Debug issues using scientific method with subagent isolation.
 **Why subagent:** Investigation burns context fast (reading files, forming hypotheses, testing). Fresh 200k context per investigation. Main context stays lean for user interaction.
 </objective>
 
+<available_agent_types>
+Valid GSD subagent types (use exact names — do not fall back to 'general-purpose'):
+- gsd-debugger — Diagnoses and fixes issues
+</available_agent_types>
+
 <context>
 User's issue: $ARGUMENTS
 

commands/gsd/research-phase.md

@@ -23,6 +23,11 @@ Research how to implement a phase. Spawns gsd-phase-researcher agent with phase
 **Why subagent:** Research burns context fast (WebSearch, Context7 queries, source verification). Fresh 200k context for investigation. Main context stays lean for user interaction.
 </objective>
 
+<available_agent_types>
+Valid GSD subagent types (use exact names — do not fall back to 'general-purpose'):
+- gsd-phase-researcher — Researches technical approaches for a phase
+</available_agent_types>
+
 <context>
 Phase number: $ARGUMENTS (required)