docs: expand markdown formatting pipeline

docs/concepts/markdown-formatting.md

@@ -3,11 +3,22 @@ summary: "Markdown formatting pipeline for outbound channels"
 read_when:
   - You are changing markdown formatting or chunking for outbound channels
   - You are adding a new channel formatter or style mapping
+  - You are debugging formatting regressions across channels
 ---
 # Markdown formatting
 
 Clawdbot formats outbound Markdown by converting it into a shared intermediate
-representation (IR) before rendering channel-specific output.
+representation (IR) before rendering channel-specific output. The IR keeps the
+source text intact while carrying style/link spans so chunking and rendering can
+stay consistent across channels.
+
+## Goals
+
+- **Consistency:** one parse step, multiple renderers.
+- **Safe chunking:** split text before rendering so inline formatting never
+  breaks across chunks.
+- **Channel fit:** map the same IR to Slack mrkdwn, Telegram HTML, and Signal
+  style ranges without re-parsing Markdown.
 
 ## Pipeline
 
@@ -22,13 +33,54 @@ representation (IR) before rendering channel-specific output.
    - **Telegram:** HTML tags (`<b>`, `<i>`, `<s>`, `<code>`, `<pre><code>`, `<a href>`).
    - **Signal:** plain text + `text-style` ranges; links become `label (url)` when label differs.
 
+## Where it is used
+
+- Slack, Telegram, and Signal outbound adapters render from the IR.
+- Other channels (WhatsApp, iMessage, MS Teams, Discord) still use plain text or
+  their own formatting rules.
+
+## Chunking rules
+
+- Chunk limits come from channel adapters/config and are applied to the IR text.
+- Code fences are preserved as a single block with a trailing newline so channels
+  render them correctly.
+- List prefixes and blockquote prefixes are part of the IR text, so chunking
+  does not split mid-prefix.
+- Inline styles (bold/italic/strike/inline-code/spoiler) are never split across
+  chunks; the renderer reopens styles inside each chunk.
+
+If you need more on chunking behavior across channels, see
+[Streaming + chunking](/concepts/streaming).
+
 ## Link policy
 
-- **Slack:** `[label](url)` -> `<url|label>`; bare URLs are left as-is.
+- **Slack:** `[label](url)` -> `<url|label>`; bare URLs remain bare. Autolink
+  is disabled during parse to avoid double-linking.
 - **Telegram:** `[label](url)` -> `<a href="url">label</a>` (HTML parse mode).
-- **Signal:** `[label](url)` -> `label (url)` unless label matches url.
+- **Signal:** `[label](url)` -> `label (url)` unless label matches the URL.
 
 ## Spoilers
 
 Spoiler markers (`||spoiler||`) are parsed only for Signal, where they map to
 SPOILER style ranges. Other channels treat them as plain text.
+
+## How to add or update a channel formatter
+
+1. **Parse once:** use the shared `markdownToIR(...)` helper with channel-appropriate
+   options (autolink, heading style, blockquote prefix).
+2. **Render:** implement a renderer with `renderMarkdownWithMarkers(...)` and a
+   style marker map (or Signal style ranges).
+3. **Chunk:** call `chunkMarkdownIR(...)` before rendering; render each chunk.
+4. **Wire adapter:** update the channel outbound adapter to use the new chunker
+   and renderer.
+5. **Test:** add or update format tests and an outbound delivery test if the
+   channel uses chunking.
+
+## Common gotchas
+
+- Slack angle-bracket tokens (`<@U123>`, `<#C123>`, `<https://...>`) must be
+  preserved; escape raw HTML safely.
+- Telegram HTML requires escaping text outside tags to avoid broken markup.
+- Signal style ranges depend on UTF-16 offsets; do not use code point offsets.
+- Preserve trailing newlines for fenced code blocks so closing markers land on
+  their own line.