Split presentation.json into per-slide files and enable parallel slide generation

[ShotaroKataoka]

Problem

Slide generation is sequential — each slide takes 1+ minute, so a 10-slide deck takes 10+ minutes. This is unacceptable for the Web UI experience. Additionally, the single presentation.json file containing all slides creates write conflicts when multiple agents work in parallel.

Proposal

1. Per-slide file splitting

Replace the monolithic presentation.json with a split structure:

decks/{deckId}/
deck.json # metadata only (template, fonts, defaultTextColor)
slides/{slug}.json # one file per slide
specs/outline.md # slug-based ordering

• deck.json is created in Phase 1 (art direction) and read-only in Phase 2
• Each slides/{slug}.json contains exactly one slide definition
• Slide ordering is determined by line order in outline.md
• presentation.json remains supported for backward compatibility in Layer 1

2. Outline slug format

Change outline format from numbered labels to slugs:

markdown

Before

• [1: Title] Audience knows the topic and speaker
• [2: Current state] Audience sees the problem

After

• [title] Audience knows the topic and speaker
• [current-state] Audience sees the problem

Slugs map directly to filenames (slides/{slug}.json). Inserting a slide — even at the front of a 50-slide deck — requires only adding one line to outline.md. No file renaming needed.

3. Agent separation (SPEC agent + Composer agent)

Split the single agent into:

• SPEC agent (Phase 1): briefing → outline → art direction. Owns user dialogue.
• Composer agent (Phase 2+3): compose + review. Stateless — reads deck.json + assigned slides + specs on each invocation.

Uses Strands Agents SDK "Agents as Tools" pattern (@tool decorator).

4. Parallel slide generation

Multiple composer agents run concurrently via asyncio.gather, each writing only its assigned slides/{slug}.json files. No write conflicts since files don't overlap.

Expected improvement: 10+ minutes → 1-2 minutes for a 10-slide deck.

Scope

Changes:

• Layer 1 (Engine): _resolve_config supports directory input (deck.json + slides/*.json)
• Layer 3 (MCP Server): Storage, generate, sandbox, init adapted to split structure
• Layer 4 (Agent): Agent separation + parallel composer
• Web UI: outlineParser.ts updated for slug format
• Workflows: outline and art-direction workflows updated

No changes to:

• MCP tool interfaces (agent-facing API stays the same)
• Layer 2 (Local MCP Server) — future follow-up

Implementation phases

1. Phase C — File splitting + serial composer agent (validate the architecture)
2. Phase B — Parallel composer agents (deliver the speed improvement)