feat: Add subagent usage guidance to refine and plan prompts

Instruct architect agents to leverage subagents for parallel work —
page analysis, codebase verification, dependency mapping, and pattern
discovery — instead of doing everything sequentially.

apps/server/agent/prompts/plan.ts

@@ -81,6 +81,15 @@ Each phase must pass: **"Could a detail agent break this into tasks without clar
 </examples>
 </specificity>
 
+<subagent_usage>
+Use subagents to parallelize your analysis — don't do everything sequentially:
+- **Domain decomposition**: Spawn separate subagents to investigate different aspects of the initiative (e.g., one for database/schema concerns, one for API surface, one for frontend components) and synthesize their findings into your phase plan.
+- **Dependency mapping**: Spawn a subagent to map existing code dependencies and file ownership while you analyze initiative requirements, so you can make informed decisions about phase boundaries and parallelism.
+- **Pattern discovery**: When the initiative touches multiple subsystems, spawn subagents to search for existing patterns in each subsystem simultaneously rather than exploring them one at a time.
+
+Don't spawn subagents for trivial initiatives with obvious structure — use judgment.
+</subagent_usage>
+
 <existing_context>
 - Account for existing phases/tasks — don't plan work already covered
 - Always generate new phase IDs — never reuse existing ones

apps/server/agent/prompts/refine.ts

@@ -33,6 +33,15 @@ Ignore style, grammar, formatting unless they cause genuine ambiguity. Rough but
 If all pages are already clear, signal done with no output files.
 </improvement_priorities>
 
+<subagent_usage>
+Use subagents to parallelize your work:
+- **Parallel page analysis**: Spawn one subagent per page (or group of related pages) to analyze clarity issues simultaneously rather than reviewing pages sequentially.
+- **Codebase verification**: When checking whether a requirement is feasible or matches existing patterns, spawn a subagent to search the codebase while you continue reviewing other pages.
+- **Cross-reference validation**: Spawn a subagent to verify that all [[page:$id|title]] cross-references are valid and consistent across pages.
+
+Don't over-split — if there are only 1-2 short pages, just do the work directly.
+</subagent_usage>
+
 <rules>
 - Ask 2-4 questions if you need clarification
 - Preserve [[page:\$id|title]] cross-references