feat: improve PRD validation robustness, fix rollback tag mismatch and add YAML frontmatter support

[vtnguyen04]

This PR introduces several improvements to make the PRD generation and validation skill more robust, accurate, and optimized for AI agent execution:

1. Structured YAML Frontmatter: Added YAML frontmatter to both comprehensive and minimal PRD templates, providing a machine-readable block at the top for AI agents to instantly parse metadata, targets, priorities, and dependency mappings.
2. Robust Heading Parsing: Fixed the heading parser in script.py to prioritize exact matches and prevent collision overlaps (e.g. preventing 'Functional Requirements' matching 'Non-Functional Requirements').
3. Safe Code Block Linting: Stripped markdown fenced and inline code blocks before scanning for vague terms to prevent false positives (e.g. matching 'secure' inside technical code configurations).
4. Flexible Acceptance Criteria: Enhanced user stories validation to support standard bulleted lists in addition to checkboxes.
5. Precise Dependency and Breakdown Checks: Cleaned Table of Contents links from validation text to prevent checks 11 and 12 from passing falsely on template instructions.
6. Rollback Tag and Safety Fix: Synced rollback tag formats between python logs and bash scripts (checkpoint-task-5 vs 005 mismatch), and added a git stash safeguard to prevent destructive resets on dirty directories.
7. Granular Gitignore: Improved setup-taskmaster.sh to safely update exclusions individually and exclude taskmaster local runtime states under .taskmaster/state/.