docs: sync make-pdf image-policy docs with final shipped behavior (v1.58.0.0)

The docs wave (87594420) predated the final review-wave commits, so two
docs drifted from shipped behavior:

- make-pdf/SKILL.md.tmpl + generated SKILL.md: remote images are BLOCKED
  with a visible placeholder (not warned-and-kept); out-of-tree reads
  (including via symlink) warn and --strict makes them fatal; --strict
  also covers oversized (>64MB) and non-regular files; troubleshooting
  entry now names the actual "[remote image blocked]" symptom.
- docs/howto-diagrams-and-formats.md: same corrections in the image
  section, CI section, and troubleshooting.
- README.md: docs/howto-diagrams-and-formats.md added to the Docs table
  (was unreachable from any entry-point doc).

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

README.md

@@ -431,6 +431,7 @@ Other references: [docs/gbrain-sync.md](docs/gbrain-sync.md) (sync-specific guid
 | Doc | What it covers |
 |-----|---------------|
 | [Skill Deep Dives](docs/skills.md) | Philosophy, examples, and workflow for every skill (includes Greptile integration) |
+| [Diagrams & Document Formats](docs/howto-diagrams-and-formats.md) | Mermaid/excalidraw fences in PDFs, image sizing and safety defaults, `--to html\|docx`, `/diagram` triplets |
 | [Builder Ethos](ETHOS.md) | Builder philosophy: Boil the Ocean, Search Before Building, three layers of knowledge |
 | [Using GBrain with GStack](USING_GBRAIN_WITH_GSTACK.md) | Every path, flag, bin helper, and troubleshooting step for `/setup-gbrain` |
 | [GBrain Sync](docs/gbrain-sync.md) | Cross-machine memory setup, privacy modes, troubleshooting |