Add epic support to speckit workflow

[IanMayo]

Problem

Large features (like Storyboarding Briefings) don't fit the current single-item backlog workflow. They need to be broken down into multiple deliverable items while maintaining traceability to the parent epic.

Proposed Solution

Add epic support to the speckit workflow:

1. BACKLOG.md Epics Section

Add a new "Epics" section above Items table:

## Epics

| ID | Title | Description | Status | Items |
|----|-------|-------------|--------|-------|
| E01 | Storyboarding Briefings | Structured briefing authoring and playback | active | 023, 024, 025 |

2. New /epic Command

Create /epic <description or doc link> that:

1. Reads the epic description (or fetches linked document)
2. Uses Opus model acting as BA + Technical Architect
3. Breaks down into deliverable backlog items
4. Each item uses Ex nomenclature in description to link back (e.g., "E01: Create storyboard schema")
5. Adds epic to Epics table, items to Items table

3. Item Nomenclature

Items from epics include epic reference:

• Description format: [E01] Item title
• Or prefix in Description column: E01: Item title

4. Epic Statuses

Status	Meaning
proposed	Epic added, not yet broken down
active	Items created, work in progress
complete	All child items complete

Success Criteria

• BACKLOG.md has Epics section with table
• /epic command exists and uses Opus for breakdown
• Command reads markdown docs (local or GitHub URLs)
• Breakdown produces 3-10 deliverable items per epic
• Items include research spikes and tech debt where needed
• Each item traces back to parent epic via Ex nomenclature
• Epic status reflects child item progress

Constraints

• Opus model required for breakdown (complex analysis)
• Items must still be independently deliverable
• Each item should move the project forward (value delivery)
• Research spikes are acceptable as items

Out of Scope

• Breaking down the Storyboarding epic (separate task after workflow exists)
• Automatic epic status updates (manual for now)
• Nested epics