Merge curated fork work: 12 new diagram types, icons, export, onboarding v2

Imports the substantive work from the aristide/diagram-design fork (PR #9)
as a curated set:

- 12 new diagram types with reference docs: radar, bar, line, gantt,
  scatter, it-state, high-level, process, medallion, data-flow,
  dp-integration, dp-security-matrix (26 types total)
- 86-icon monochrome IT/cloud primitive, build script, and gallery
- /diagram-design:export command with PNG/SVG export reference
- Onboarding v2: URL, skill, and folder design-token sources
- Mandatory connector rules (orthogonal elbows, label gaps, fanned
  attach points, bridge crossings) + architecture/ER connector upgrades
- Series palette tokens for multi-series chart types
- THIRD_PARTY_LICENSES.md covering Tabler, Simple Icons, log-z, Devicon

Not taken from the fork: plugin-manifest identity changes, the client-brand
takeover of style-guide.md, and client-branded example files. The six
affected type examples were regenerated in the neutral editorial skin with
generic content, and their README screenshots recaptured. Reference docs
for those types were rewritten to semantic tokens and typography roles.

Version: 1.0.0 -> 2.0.0.

Co-authored-by: Aristide <mendoo.aristide@gmail.com>

commands/export-diagram.md

@@ -0,0 +1,36 @@
+---
+description: Export a diagram-design HTML file to .svg and .png next to the source
+argument-hint: <html-file> [--svg-only|--png-only] [--scale=N] [--output=<path>]
+allowed-tools:
+  - Read
+  - Write
+  - Edit
+  - Bash
+  - Glob
+---
+
+Export the diagram HTML at `$1` to `.svg` and/or `.png`, following the procedure documented in [`skills/diagram-design/references/export.md`](../skills/diagram-design/references/export.md). Treat that reference as the source of truth — don't reimplement the logic here.
+
+Full argument string: `$ARGUMENTS`
+
+## Defaults
+
+- Produce **both** `.svg` and `.png` next to the source (e.g. `diagram.html` → `diagram.svg` + `diagram.png`).
+- PNG renders at `device_scale_factor=2`.
+
+## Flags
+
+- `--svg-only` — emit only the SVG. Skip Playwright entirely.
+- `--png-only` — emit only the PNG.
+- `--scale=1` / `--scale=2` / `--scale=3` — override the PNG device scale factor. Default `2`.
+- `--output=<path>` — override the output base path; the format extension is appended. Applies to both formats when both are produced.
+
+## Required behaviour
+
+1. **No source path provided** → ask the user which `.html` file to export. Don't guess.
+2. **Source is `assets/index.html`** (the gallery, multiple SVGs in one file) → refuse and ask which specific diagram file. Per the reference's edge-case section.
+3. **Source has no `<svg>` block** → refuse and tell the user; don't write anything.
+4. **PNG requested but Playwright not installed** → surface the install instruction from the reference verbatim and stop. Do **not** auto-install.
+5. **PNG requested with `--scale` outside {1,2,3}** → reject; valid values are 1, 2, 3.
+
+After producing the outputs, report the file paths and sizes back to the user.