fix: make template metadata line breaks markdownlint-safe

[WOLIKIMCHENG]

Summary

• Replace trailing-space Markdown hard breaks in core templates with explicit blank lines
• Separate template metadata fields so markdownlint/editor auto-fix does not collapse them into one rendered paragraph
• Clean trailing whitespace in the affected templates

Fixes #1343

Validation

• git diff --check -- templates/spec-template.md templates/plan-template.md templates/tasks-template.md
• rg -n " $" templates/spec-template.md templates/plan-template.md templates/tasks-template.md
• Temporary preview/paragraph assertion for key template fields
• Markdown preview screenshot generated locally
  cache