docs: update task brief and Claude instructions for improved planning guidance

.agent/templates/task-brief.md

@@ -4,6 +4,11 @@
 
 `YYYY-MM-DD-short-name`
 
+## Related PRD sections
+
+- ...
+- ...
+
 ## Goal
 
 Describe the intended change in one or two sentences.

.claude/skills/plan-task/SKILL.md

@@ -6,7 +6,13 @@ disallowed-tools: Write Edit MultiEdit
 
 You are planning a coding task.
 
-Read the task brief provided by the user. If a `.agent/tasks/<task-id>/brief.md` file is mentioned, read it first.
+Before producing the implementation plan:
+
+1. Read PRD.md.
+2. Read the task brief provided by the user. If a `.agent/tasks/<task-id>/brief.md` file is mentioned, read it first.
+3. Identify which PRD sections are affected.
+4. Verify that the task is consistent with the PRD.
+5. Report any ambiguity or conflict.
 
 Workflow:
 

CLAUDE.md

@@ -1,5 +1,25 @@
 # Claude Instructions
 
+## Project Documentation
+
+The product requirements are defined in `PRD.md`.
+
+When planning or implementing features:
+- Treat `PRD.md` as the source of truth for product behavior.
+- If a task conflicts with `PRD.md`, stop and report the conflict.
+- If a task introduces behavior not covered by `PRD.md`, explicitly identify the gap.
+
+Documentation priority:
+
+1. Task Brief
+2. PRD.md
+3. Existing code
+
+If there is a conflict:
+Task Brief > PRD.md > Existing code
+
+Always report conflicts before implementing.
+
 ## Working style
 
 - Prefer small, focused changes.
@@ -8,6 +28,9 @@
 - For non-trivial tasks, plan before editing.
 - If no code change is needed, stop and explain why with evidence.
 - If requirements are ambiguous, list assumptions and choose the safest minimal default.
+- If any decision is not clearly instructed, stop and ask the user before proceeding.
+- When resolving Ruby, Rails, or Tailwind CSS questions, use the Context7 MCP tool — do not rely on training data.
+- Always use `bin/rails generate` to create artifacts (models, controllers, migrations, etc.) instead of crafting files manually.
 - Keep generated artifacts in `.agent/tasks/<task-id>/` when the task is medium or larger.
 
 ## Task lanes