docs: apply Codex doc-review findings for v1.58.0.0

Cross-model doc review (Codex, read-only) checked the v1.58.0.0 docs
against the shipped code. Fixes:

- howto + make-pdf SKILL: diagram source is preserved base64 in a
  data-gstack-source attribute, not an HTML comment (-- in mermaid
  arrows would corrupt a comment); fences must start at column 0;
  fence options example gains page=portrait; --to html "zero network
  refs" qualified (--allow-network deliberately keeps remote tags).
- /diagram description, README + docs/skills.md rows: the hand-drawn
  aesthetic belongs to the .excalidraw artifact; rendered SVG/PNG use
  mermaid's clean neutral theme (lib/diagram-render entry.ts pins
  theme: "neutral").
- CHANGELOG v1.58.0.0 wording: --strict coverage lists all five fatal
  classes (missing/remote/out-of-tree/oversized/non-regular); fences
  are vector SVG in pdf+html, 300dpi PNG in docx; hand-drawn claim
  scoped to the .excalidraw file.
- lib/diagram-render/README: Page API table gains __downscaleRaster.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

scripts/proactive-suggestions.json

@@ -80,7 +80,7 @@
     },
     "diagram": {
       "lead": "Turn an English description (or mermaid source) into a diagram triplet: the source, an editable .excalidraw file you can open",
-      "routing": "on excalidraw.com,\nand rendered SVG + PNG. Hand-drawn excalidraw aesthetic, fully offline.\nUse when asked to \"make a diagram\", \"draw the architecture\", \"create a\nflowchart\", \"diagram this\", or \"visualize this flow\".",
+      "routing": "on excalidraw.com,\nand rendered SVG + PNG (clean mermaid style; the .excalidraw carries the\nhand-drawn aesthetic). Fully offline.\nUse when asked to \"make a diagram\", \"draw the architecture\", \"create a\nflowchart\", \"diagram this\", or \"visualize this flow\".",
       "voice_line": null
     },
     "document-generate": {