feat: add child actions menu and achievements feature design

- Introduced a new feature specification for displaying a menu with actions (Award Certificate, Award Badge) when a child's card is clicked in ParentView.
- Created a detailed plan for an achievements system, outlining phases for implementation, including achievement taxonomy, MVP set, and UX considerations.
- Added a template for future feature specifications to standardize documentation.

.opencode/skills/feature-pipeline/SKILL.md

@@ -0,0 +1,23 @@
+---
+name: feature-pipeline
+description: "Executes the full dev-to-docs pipeline: Developer -> Tester -> Reviewer -> Writer."
+---
+
+## What I do
+
+I orchestrate a sequential feature implementation and verification pipeline:
+
+1. **Developer**: Implements the feature based on the spec.
+2. **Reviewer**: Audits the code and architectural soundness.
+3. **Tester**: Runs full unit/E2E test suites; repairs failures if found.
+4. **Writer**: Updates README and API docs based on verified code.
+
+## Execution Rules
+
+- Stop and ask the user for clarification if a step fails or is ambiguous.
+- Use the `@` mention to trigger the respective subagents sequentially.
+- Pass the context from the previous stage to the next stage to ensure consistency.
+
+## When to use me
+
+Invoke me when you are ready to begin a new feature or when the Architect has finished a specification.