fix: Use blank lines in template headers to prevent markdownlint stripping line breaks

[echarrod]

Summary

Fixes template header fields rendering on a single line when trailing spaces are stripped by markdownlint (MD009 rule) or similar tools.

Before: Header fields use trailing double spaces for line breaks, which get stripped by linters
After: Header fields use blank lines, which are immune to linter auto-fix

Root Cause

This issue is caused by markdownlint's MD009 rule (no-trailing-spaces) which strips trailing whitespace from Markdown files.

markdownlint is widely integrated across the ecosystem:

Tool	Integration
VS Code	vscode-markdownlint extension
Sublime Text	markdownlint plugin
Vim/Neovim	coc-markdownlint extension
Emacs	flymake-markdownlint-cli2
GitHub Actions	markdownlint-cli2

Why blank lines are the better solution

Using blank lines instead of trailing spaces:

• Is immune to linter auto-fix behavior
• Is visually obvious in source files
• Works consistently across all Markdown parsers
• Follows the principle of explicit over implicit formatting

Test plan

• View spec-template.md in GitHub - header fields should render on separate lines
• View plan-template.md in GitHub - header fields should render on separate lines
• View tasks-template.md in GitHub - header fields should render on separate lines
• Verify in JetBrains IDE markdown preview
• Run markdownlint on templates - no MD009 violations

Fixes #1343

🤖 Generated with Claude Code

[Copilot]

Pull request overview

This PR fixes markdown rendering issues in template files by replacing trailing double-space line breaks with blank lines, ensuring header fields render correctly on separate lines in GitHub and JetBrains IDEs.

Key Changes:

• Removes fragile trailing double-space line breaks from template header sections
• Adds explicit blank lines between header fields for consistent cross-platform rendering

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated no comments.

File	Description
templates/spec-template.md	Removes trailing double spaces from Feature Branch, Created, and Status fields and adds blank lines after each for proper separation
templates/plan-template.md	Adds blank line after the Branch/Date/Spec header field to ensure proper rendering
templates/tasks-template.md	Adds blank line after the Input field to maintain consistency with other template headers

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The changes are well-focused and directly address the rendering issue described in the PR. The modifications follow a consistent pattern across all three template files, replacing the markdown convention of trailing double spaces (which can be invisible and fragile) with explicit blank lines for better maintainability and cross-platform rendering compatibility.

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[wmalgadey]

maybe a better approach would be to add double spaces at the end of the line? this would not break linting as well and would be a nicer visual IMO

**Feature Branch**: `[###-feature-name]`  
**Created**: [DATE]  
**Status**: Draft  

[echarrod]

I agree it looks slightly nicer, but I outlined in the source issue (#1343) that markdownlint (used by many tools) removes the double spaces from these lines automatically