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

[Copilot]

Pull request overview

This PR updates the core Markdown templates to avoid relying on trailing-space hard breaks for rendering template “metadata” fields, making them resilient to markdownlint/auto-formatters that strip trailing whitespace (fixes #1343).

Changes:

• Replaced trailing-double-space hard breaks with explicit blank lines between template header/metadata fields.
• Removed trailing whitespace in the affected templates (including within comment blocks).
• Adjusted formatting so key template fields render as separate paragraphs across common Markdown parsers/editors.

Show a summary per file

File	Description
templates/spec-template.md	Splits header metadata fields into separate paragraphs and removes trailing-space hard breaks.
templates/plan-template.md	Adds blank lines between technical-context metadata fields to prevent paragraph collapsing when trailing spaces are stripped.
templates/tasks-template.md	Adds spacing after the Input field and cleans up whitespace in the instructional comment block.

Copilot's findings

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• Files reviewed: 3/3 changed files
• Comments generated: 0