Unify OpenCode and Codex starter kits

- Add .shared/prompts/ with tool-neutral prompts
- Add .opencode/config/project.md, rules/, skills/ from opencode-kit
- Add .codex/config/project.md from codex-kit
- Copy .shared/prompts/ to both .opencode/prompts/ and .codex/prompts/
- Add features/INDEX.md and features/README.md
- Update README with unified structure

The two original starter kits are now redundant.

.opencode/skills/requirements/SKILL.md

@@ -0,0 +1,90 @@
+---
+name: requirements
+description: Creates feature specs with user stories, acceptance criteria, and edge cases
+category: development
+---
+
+# Requirements Engineer
+
+Creates detailed feature specifications for the AI Coding Starter Kit workflow.
+
+## When to Use
+
+Run this skill when you want to add a new feature to the project. It:
+1. Reads the existing PRD to understand project context
+2. Checks the feature index for the next available ID
+3. Creates a feature spec with user stories, acceptance criteria, and edge cases
+4. Updates the feature tracking index
+
+## Usage
+
+```
+/requirements
+```
+
+Then describe your feature idea. The skill will ask follow-up questions to clarify scope.
+
+## Skill Steps
+
+1. **Read project context**
+   - Read `docs/PRD.md` for project vision and constraints
+   - Read `features/INDEX.md` to see existing features and next PROJ-X ID
+
+2. **Clarify requirements** (interactive)
+   - Ask about target users
+   - Ask about core functionality
+   - Ask about MVP scope
+   - Ask about edge cases to handle
+
+3. **Create feature spec**
+   - Create file `features/PROJ-X-feature-name.md`
+   - Include: Feature name, ID, user stories, acceptance criteria, edge cases, dependencies
+
+4. **Update tracking**
+   - Update `features/INDEX.md` with new feature status (Pending)
+
+5. **Suggest next step**
+   - Recommend running `/architecture` next
+
+## Output
+
+- `features/PROJ-X-feature-name.md` - The feature specification
+- Updated `features/INDEX.md` - Feature tracking updated
+
+## Feature Spec Format
+
+```markdown
+# Feature: [Name]
+
+## ID
+PROJ-X
+
+## Status
+Pending
+
+## User Stories
+- As a [role], I want [action], so that [benefit]
+- ...
+
+## Acceptance Criteria
+- [ ] Criterion 1 (testable)
+- [ ] Criterion 2 (testable)
+
+## Edge Cases
+- Case 1: description
+- Case 2: description
+
+## Dependencies
+- PROJ-Y (required first)
+- ...
+
+## Tech Design
+(Added later by /architecture skill)
+```
+
+## Context Recovery
+
+If context is compacted mid-task:
+1. Re-read `docs/PRD.md` and `features/INDEX.md`
+2. Check if feature spec was already created
+3. Continue from where you left off