fix(prompts): change agent: to mode: in frontmatter

Fixed 22 prompt files:
- Changed 'agent:' to 'mode:' (correct frontmatter key)
- Removed duplicate 'search' entries from tools arrays

This aligns with prompt authoring rules in prompt.instructions.md

.github/prompts/create-github-issues-feature-from-implementation-plan.prompt.md

@@ -1,5 +1,5 @@
 ---
-agent: 'agent'
+mode: 'agent'
 description: 'Create GitHub Issues from implementation plan phases using feature_request.yml or chore_request.yml templates.'
 tools: ['search/codebase', 'search', 'github', 'create_issue', 'search_issues', 'update_issue']
 ---