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>

commands/doctor.md

@@ -0,0 +1,22 @@
+---
+description: Run one-shot environment diagnostics for Diagram Design readiness
+argument-hint: [--strict] [--json]
+allowed-tools:
+  - Read
+  - Bash
+  - Glob
+---
+
+Run environment diagnostics for Diagram Design by following [`skills/diagram-design/references/doctor.md`](../skills/diagram-design/references/doctor.md). Treat that reference as the source of truth and do not reimplement its logic here.
+
+Full argument string: `$ARGUMENTS`
+
+## Required behavior
+
+1. Locate and apply the exact checks and output contract from `references/doctor.md`.
+2. Keep this command read-only: do not install dependencies, do not edit files, and do not run destructive git operations.
+3. If a check command fails, capture stderr, mark the check `warn` or `fail` per the reference, and continue remaining checks.
+4. Print the compact summary line plus per-check statuses, and include `Next actions` only when needed.
+5. If `--json` is passed, append the structured JSON block defined by the reference.
+
+Report only verified results from this run.