feat: add presentation creation tool

[avoidwork]

Summary

Add a tool to create PowerPoint presentations (.pptx) from structured content — slides, layouts, formatting, and media.

Motivation

The existing PPTX tool (src/tools/fileExtract/pptx.js) is read-only — it extracts content from existing presentations. Marketing teams live in slide decks: creating pitch decks, status reports, training materials, and client presentations. Currently the agent can only read PPTX files, not create them. Users must fall back to shell commands (python-pptx, libreoffice) or manual creation, which is fragile and loses formatting fidelity.

Proposed Solution

Create a presentation creation tool that accepts structured content and generates .pptx files:

• Slide creation: Add slides with titles, bullet points, images, charts
• Layout support: Title slide, content slide, two-column, comparison, quote, image-only layouts
• Formatting: Font styles, colors, alignment, spacing
• Media: Embed images, charts, tables
• Template support: Apply existing PPTX templates or create from scratch
• Output: Save as .pptx file path

Each tool should follow the existing tool pattern in src/tools/ — a zod schema, an impl function, and registration in index.js with filesystem:write permission.

Alternatives Considered

• Shell-based tools (python-pptx, libreoffice): fragile, requires user-installed dependencies, no structured input.
• Read-only PPTX extraction: insufficient for creation workflows.
• Web-based presentation service: adds infrastructure complexity for a local-first tool.

OpenSpec Note

This project uses OpenSpec for feature development. If this request is approved, I will:

1. Run /opsx:propose to generate a full proposal with specs and tasks
2. Iterate on the design before any code is written
3. Follow the task-driven implementation workflow

Additional Context

This should integrate with the document creation gap (#778) and text processing gap (#784) — e.g., generating marketing copy and then creating a slide deck from it. The existing pptx.js read tool (src/tools/fileExtract/pptx.js) would serve as the input layer for reading existing presentations as templates.

Dependencies

• Primary library: pptxgenjs (v3.x+) — pure JavaScript, no system dependencies, cross-platform, supports layouts, images, charts, fonts, and templates. Actively maintained with 2M+ weekly npm downloads.
• Alternative considered: node-pptx — less maintained, fewer features, no chart support.

Testing Strategy

• Unit tests: Verify slide creation with various layouts, font rendering, image embedding, and chart generation using pptxgenjs API mocks.
• Integration test: Generate a .pptx file, verify it opens correctly (checksum validation against known-good output).
• Edge cases: Empty content, very long text (truncation/overflow), unsupported image formats, missing font fallback.
• No real file needed in CI: Generate to a temp directory and validate structure (zip archive with expected XML parts).

Security Considerations

• File output path must be validated against the allowed write directory (per AGENTS.md 1.2).
• Image files must pass MIME whitelist validation before embedding (no executable payloads disguised as images).
• Template files loaded from disk must be validated as valid PPTX (zip structure check) before use.

Implementation Notes

• Mirror the existing read tool structure: src/tools/fileCreate/pptx.js (or extend pptx.js if the read/write split is acceptable).
• Zod schema should accept a structured object: { title, slides: [{ layout, title, content, images, charts }] }.
• Font handling: use built-in fonts (Arial, Calibri, Helvetica) by default; allow custom font file upload for advanced use.
• Chart support: start with basic bar/line/pie charts; complex charts can be deferred to a follow-up PR.
• Template support: accept a path to an existing .pptx file and clone its master slides as the base for new content.