fix: Increase detail agent task sizing to reduce tiny-task overhead

Each execute agent has significant startup cost (context loading, codebase
exploration, baseline tests). The previous sizing guidance (sweet spot
<150 lines, 1-3 files, merge at <20 lines) produced tasks too small to
justify that overhead.

New guidance targets cohesive feature units: 200-500 lines across 3-6
files as the sweet spot, merge threshold at <100 lines, and explicit
instruction to bundle related changes (validation + route + tests = one
task, not three).

apps/server/agent/prompts/detail.ts

@@ -32,23 +32,39 @@ Every task body must include:
 4. **Verification command** — exact command to confirm completion
 
 <examples>
-<example label="bad">
+<example label="bad — too vague">
 Title: Add user validation
 Body: Add validation to the user model. Make sure all fields are validated properly.
 </example>
-<example label="good">
+<example label="bad — too small (would be ~40 lines, not worth agent overhead)">
 Title: Add Zod validation schema for user creation
-Body: Create \`src/api/validators/user.ts\` — Zod schema for CreateUserInput:
-- email: valid format, lowercase, max 255 chars
-- name: 1-100 chars, trimmed
-- password: min 8 chars, uppercase + number required
+Body: Create \`src/api/validators/user.ts\` — Zod schema for CreateUserInput.
+Files: src/api/validators/user.ts (create)
+</example>
+<example label="good — cohesive feature unit">
+Title: Add user creation API endpoint with validation and tests
+Body: Implement the complete user creation flow:
 
-Test file: \`src/api/validators/user.test.ts\`
-Tests: valid input passes, missing fields rejected, invalid email rejected,
-  weak password rejected, whitespace-only name rejected
+**Validation** — Create \`src/api/validators/user.ts\`:
+- Zod schema for CreateUserInput: email (valid format, lowercase, max 255), name (1-100 chars, trimmed), password (min 8, uppercase + number required)
+- Export \`validateCreateUser()\` wrapper that returns typed result
 
-Files: src/api/validators/user.ts (create), user.test.ts (create)
-Verify: \`npm test -- src/api/validators/user.test.ts\`
+**Repository** — Create \`src/db/repositories/user-repository.ts\`:
+- \`createUser(input: CreateUserInput): Promise<User>\` — insert with bcrypt-hashed password
+- \`findByEmail(email: string): Promise<User | null>\` — for duplicate checking
+
+**API Route** — Create \`src/api/routes/users.ts\`:
+- POST /api/users — validate input, check email uniqueness (409 if duplicate), create user, return 201 with user (no password)
+- Wire into existing Express router in \`src/api/index.ts\`
+
+**Tests** — Create \`src/api/routes/users.test.ts\`:
+- Happy path: valid input → 201 + user object without password field
+- Validation: missing fields → 400, invalid email → 400, weak password → 400
+- Duplicate: existing email → 409
+- Edge cases: whitespace-only name → 400, email case normalization
+
+Files: src/api/validators/user.ts (create), src/db/repositories/user-repository.ts (create), src/api/routes/users.ts (create), src/api/routes/users.test.ts (create), src/api/index.ts (modify)
+Verify: \`npm test -- src/api/routes/users.test.ts\`
 </example>
 </examples>
 </task_body_requirements>
@@ -63,11 +79,17 @@ Tasks with no dependencies run in parallel. Add a dependency when one task needs
 </file_ownership>
 
 <task_sizing>
-- **<150 lines, 1-3 files**: Sweet spot
-- **150-300 lines, 4-5 files**: Only for mechanical/boilerplate work with precise specs
-- **300+ lines or 5+ files**: Split it
-- **<20 lines**: Merge with a related task
+Each task is handled by a separate agent that must load the full codebase context, read input files, explore patterns, and run tests — significant overhead per task. Size tasks to justify that cost.
+
+**Target: cohesive feature units, not atomic file operations.**
+
+- **Sweet spot: 200-500 lines, 3-6 files** — a complete vertical slice (e.g., schema + repository + route + validation + tests)
+- **Acceptable: 500-800 lines, 6-8 files** — larger features with clear scope and precise specs
+- **Split when: 800+ lines or 8+ files** — genuinely independent subfeatures
+- **Merge when: <100 lines or single-file changes** — fold into a related task; a standalone 30-line config tweak doesn't justify agent startup
 - **1 sentence description**: Too vague — add detail or merge
+
+Bundle related changes into one task. "Add user validation" + "Add user API route" + "Add user route tests" is ONE task ("Add user creation endpoint with validation and tests"), not three.
 </task_sizing>
 
 <checkpoint_tasks>
@@ -92,7 +114,7 @@ Before signal.json "done":
 - [ ] Every task has a file ownership list
 - [ ] No parallel tasks share files
 - [ ] Every task is executable without clarifying questions
-- [ ] Tasks sized within ~20-300 lines changed
+- [ ] Tasks sized within ~100-800 lines changed (no single-file throwaway tasks)
 - [ ] No duplicates with existing context tasks
 </definition_of_done>`;
 }