feat: consolidate reviewed Diagram Design 2.6.0 release (#134)

* fix(commands): stop hardcoding the visual-type count

Integrates the final reviewed diff from PR #112.

* docs(high-level): correct effective width checklist

Integrates the final reviewed diff from PR #105.

* fix(commands): quote profile argument hint

Integrates the final reviewed diff from PR #126.

* test(sankey): gate conservation and ribbon geometry

Integrates the final reviewed diff from PR #103.

* feat(tooling): add doctor diagnostics gate

Integrates the final reviewed diff from PR #132.

* feat(scripts): lint diagrams as rendered

Integrates the final reviewed diff from PR #18.

* feat(types): add quantitative polar chart

Integrates the final reviewed diff from PR #101.

* chore(release): consolidate reviewed changes in 2.6.0

Documents every combined gate and synchronizes Claude, Codex, and Factory metadata for the 39-type release.

* fix(doctor): support installed-skill projects

---------

Co-authored-by: mysoniw <soniw1@gmail.com>
Co-authored-by: thang <76550342+thangldw@users.noreply.github.com>
Co-authored-by: YinShaokun <yinshaokun@didiglobal.com>
Co-authored-by: 0xDarkMatter <0xDarkMatter@users.noreply.github.com>
Co-authored-by: Rupam Pal <rupam.pal@euromonitor.com>
Co-authored-by: Yuvraj3905 <yuvraj202001@gmail.com>

docs/screenshots/manifest.json

@@ -101,6 +101,15 @@
       "width": 2400,
       "height": 1202
     },
+    {
+      "slug": "polar",
+      "source": "skills/diagram-design/assets/example-polar.html",
+      "screenshot": "docs/screenshots/polar.png",
+      "source_sha256": "596e2042f18044ef1fa2c2eb849f39fa5c7767b2a17e272617ebd27e10bc86b1",
+      "screenshot_sha256": "a1dd1d4cb38db2217759c5d256cc56d832acc4f3077f090cc8691d6e4f82ef31",
+      "width": 2400,
+      "height": 1202
+    },
     {
       "slug": "loop",
       "source": "skills/diagram-design/assets/example-loop.html",