Feature: PushMD - Media upload & upsert support for push-only staging workflow

### Summary / Context

Currently, Push MD allows Markdown-first teams and AI coding agents to clone, edit, and push WordPress posts, pages, and templates. However, there is no support for managing post image assets through Git. When an author or agent creates a new post locally that includes diagrams, screenshots, quote cards, or a featured image, they must manually upload those images via WP-Admin and copy the resulting URLs back into Markdown before pushing.

I propose adding **native Git media support to Push MD** using a **push-only staging directory (`media/`)**. Authors can drop image binaries into `media/`, reference them using relative paths in Markdown or Gutenberg blocks, and push via Git. Push MD handles fail-closed image validation, uploads/upserts the files into the WordPress Media Library, rewrites all inline content & block references to absolute WordPress attachment URLs, and cleans up the staging files on `git pull` to keep local checkouts lightweight.

### Why Media Upload Support is Useful

1. **Complete Offline & Agent Workflows**: Coding agents and local Markdown editors (Obsidian, VS Code, Cursor) can create complete posts - text and images - in a single Git commit without requiring manual WP-Admin file uploads.
2. **Lean Local Repositories (Push-Only Staging)**: By operating `media/` as a push-only staging area, binary image blobs do not accumulate in Git history or bloat local checkouts over time.
3. **In-Place Image Versioning**: Updating an existing image via Git overwrites the file on disk and regenerates WordPress image metadata without creating duplicate attachment records (`image-1.png`, `image-2.png`).
4. **Automatic Attachment Metadata**: Alt text, image titles, and `` captions written in Markdown or HTML automatically populate WordPress attachment metadata.

### Proposed Architecture & Workflow

#### 1. The `media/` Staging Directory
- A `media/` directory is present in the repository tree (maintained via a `media/.gitkeep` placeholder).
- Authors place image files directly under `media/` (e.g. `media/architecture-diagram.png`).
- In post Markdown, images are referenced using standard relative syntax:

```markdown
---
title: "New Product Launch"
featured_image: "../media/hero.webp"
---
```

#### 2. Push-Time Ingestion & URL Rewriting
When a commit is pushed to `trunk`:
1. **Upfront Payload Ingestion**: Push MD detects files in `media/`, validates them, and uploads them to the WordPress Uploads folder, creating or updating attachment records.
2. **Contextual Metadata Assignment**: Alt text (``), titles (`"Title Text"`), and figure captions (`...`) in post markup populate `_wp_attachment_image_alt`, `post_title`, and `post_excerpt` on the corresponding attachment.
3. **Multi-Format Content Rewriting**: Relative `media/` paths are rewritten to absolute WordPress upload URLs across:
- Markdown image syntax: ``
- HTML attributes: `src`, `href`, `poster`, `srcset` across ``, ``, ``, and `` tags
- Gutenberg Block comment JSON metadata: `` (updates `id`, `mediaId`, and `url` fields)
- Front matter `featured_image` (assigns post thumbnail attachment ID)
4. **Filename-Based Re-Linking**: If a Markdown post is pushed in a later commit referencing `media/existing-chart.png` (where the binary is no longer in the commit tree), Push MD resolves `existing-chart.png` against existing Media Library attachments by filename and links it cleanly.

### Why not WordPress media REST API?
I implemented this using REST API initially, but that requires additional maintenance, authentication and integration. Doing it via PushMD is much cleaner approach.

Start with the push synchronization path and the mentioned export_media_content() behavior, then trace how WordPress uploads are handled. Use the proposed media/ workflow, validation rules, URL rewriting, upsert behavior, and pull cleanup as acceptance criteria; the issue names no implementation files or tests, so project discovery is required.

Written by the indexing model from the issue text.