chore(style): remove all em-dashes from user-facing docs

Final sweep to drop every U+2014 em-dash character from the site'"'"'s
user-facing surfaces. Replaces em-dash with colon (:) globally. The
colon is the natural substitute for an em-dash used as a separator or
clarifier, and it preserves the rhythm of titles and section markers
without the AI-tell rhythm crutch.

Scope (29 files):
- README.md, CONTRIBUTING.md, GLOSSARY.md
- en/src/*.md (ch01-ch10 + SUMMARY + index)
- vi/src/*.md (ch01-ch10 + SUMMARY + index)
- zh/src/SUMMARY.md (chapter titles only)
- assets/README.md (logo pack list)
- en/vi/zh/theme/custom.css (CSS comments)
- scripts/add-missing-headings.py, backport-mermaid-ch01.py (docstrings)

Skipped:
- zh/src/ch*.md — canonical source from dgzhuya.com (author uses
  Chinese typographic —— as section separator; preserve fidelity)
- docs/superpowers/{specs,plans}/* — historical design docs
- scripts/translations/*.json, scripts/fetch-zh.ps1 — regenerated by
  fetch-zh.ps1 from the source

Verify:
- Build clean (no unclosed-tag warnings introduced; backticks in code
  spans like `Model<TApi>` preserved)
- Frontmatter validation: 30 files OK
- Sync check: 10 chapters in sync across 3 langs

Author: hypnguyen1209

assets/README.md

@@ -1,4 +1,4 @@
-# Pi Docs — Brand Assets
+# Pi Docs : Brand Assets
 
 This directory holds the official **Pify** brand pack used across the docs site
 (favicon, sidebar logo, hero, GitHub social preview, README banner).
@@ -25,12 +25,12 @@ machine). All assets here were copied from that source.
 
 ## Where each is used
 
-- `favicon.svg` / `favicon.png` — browser tab icon (per mdBook `theme/favicon.{svg,png}`)
-- `pify-logo-light-bg.svg` — hero on each language `index.md`, README banner
-- `pify-logo-dark-bg.svg` — reserved for dark-only contexts (not currently wired)
-- `pify-light-512.png` / `pify-dark-512.png` — sidebar logo via `theme/custom.css`
+- `favicon.svg` / `favicon.png` : browser tab icon (per mdBook `theme/favicon.{svg,png}`)
+- `pify-logo-light-bg.svg` : hero on each language `index.md`, README banner
+- `pify-logo-dark-bg.svg` : reserved for dark-only contexts (not currently wired)
+- `pify-light-512.png` / `pify-dark-512.png` : sidebar logo via `theme/custom.css`
   (light variant by default, dark variant in `navy` / `coal` / `ayu` / `rust` themes)
-- `social-preview.png` (in `.github/`) — GitHub social preview card (1280×640)
+- `social-preview.png` (in `.github/`) : GitHub social preview card (1280×640)
 
 ## Design tokens
 

en/src/SUMMARY.md

@@ -4,10 +4,10 @@
 
 # Chapters
 
-- [Chapter 1: Introduction — Why Pi-Agent Is Worth Your Time](./ch01-overview.md)
+- [Chapter 1: Introduction : Why Pi-Agent Is Worth Your Time](./ch01-overview.md)
 - [Chapter 2: Three-Layer Architecture](./ch02-three-layer-arch.md)
 - [Chapter 3: Agent Loop](./ch03-agent-loop.md)
-- [Chapter 4: Model Invocation — One Line, Many Providers](./ch04-model-invocation.md)
+- [Chapter 4: Model Invocation : One Line, Many Providers](./ch04-model-invocation.md)
 - [Chapter 5: Tool System](./ch05-tool-system.md)
 - [Chapter 6: Message System](./ch06-messages.md)
 - [Chapter 7: Event-Driven Architecture](./ch07-event-driven.md)