feat(matrix): render bot messages with HTML formatting

Add HTML formatted_body to Matrix bot messages so that markdown-style
formatting (code blocks, bold, italic, lists) renders properly in Matrix
clients. Uses the pulldown-cmark crate to convert markdown to HTML and
sets the message format to org.matrix.custom.html.

Story: 188_story_render_matrix_bot_messages_with_html_formatting

Cargo.lock

@@ -2766,6 +2766,24 @@ dependencies = [
  "syn 2.0.116",
 ]
 
+[[package]]
+name = "pulldown-cmark"
+version = "0.12.2"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "f86ba2052aebccc42cbbb3ed234b8b13ce76f75c3551a303cb2bcffcff12bb14"
+dependencies = [
+ "bitflags 2.11.0",
+ "memchr",
+ "pulldown-cmark-escape",
+ "unicase",
+]
+
+[[package]]
+name = "pulldown-cmark-escape"
+version = "0.11.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "007d8adb5ddab6f8e3f491ac63566a7d5002cc7ed73901f72057943fa71ae1ae"
+
 [[package]]
 name = "quick-xml"
 version = "0.36.2"
@@ -3771,6 +3789,7 @@ dependencies = [
  "poem",
  "poem-openapi",
  "portable-pty",
+ "pulldown-cmark",
  "reqwest 0.13.2",
  "rust-embed",
  "serde",

Cargo.toml

@@ -33,3 +33,4 @@ matrix-sdk = { version = "0.16.0", default-features = false, features = [
     "native-tls",
     "sqlite",
 ] }
+pulldown-cmark = { version = "0.12", default-features = false, features = ["html"] }

server/Cargo.toml

@@ -29,6 +29,7 @@ toml = { workspace = true }
 uuid = { workspace = true, features = ["v4", "serde"] }
 walkdir = { workspace = true }
 matrix-sdk = { workspace = true }
+pulldown-cmark = { workspace = true }
 
 [dev-dependencies]
 tempfile = { workspace = true }

server/src/matrix/bot.rs

@@ -1,5 +1,6 @@
 use crate::llm::providers::claude_code::{ClaudeCodeProvider, ClaudeCodeResult};
 use crate::slog;
+use pulldown_cmark::{Options, Parser, html};
 use matrix_sdk::{
     Client,
     config::SyncSettings,
@@ -293,8 +294,9 @@ async fn handle_message(
     let post_room = room.clone();
     let post_task = tokio::spawn(async move {
         while let Some(chunk) = msg_rx.recv().await {
+            let html = markdown_to_html(&chunk);
             let _ = post_room
-                .send(RoomMessageEventContent::text_plain(chunk))
+                .send(RoomMessageEventContent::text_html(chunk, html))
                 .await;
         }
     });
@@ -388,6 +390,26 @@ async fn handle_message(
     }
 }
 
+// ---------------------------------------------------------------------------
+// Markdown rendering helper
+// ---------------------------------------------------------------------------
+
+/// Convert a Markdown string to an HTML string using pulldown-cmark.
+///
+/// Enables the standard extension set (tables, footnotes, strikethrough,
+/// tasklists) so that common Markdown constructs render correctly in Matrix
+/// clients such as Element.
+pub fn markdown_to_html(markdown: &str) -> String {
+    let options = Options::ENABLE_TABLES
+        | Options::ENABLE_FOOTNOTES
+        | Options::ENABLE_STRIKETHROUGH
+        | Options::ENABLE_TASKLISTS;
+    let parser = Parser::new_ext(markdown, options);
+    let mut html_output = String::new();
+    html::push_html(&mut html_output, parser);
+    html_output
+}
+
 // ---------------------------------------------------------------------------
 // Paragraph buffering helper
 // ---------------------------------------------------------------------------
@@ -417,6 +439,43 @@ pub fn drain_complete_paragraphs(buffer: &mut String) -> Vec<String> {
 mod tests {
     use super::*;
 
+    // -- markdown_to_html ---------------------------------------------------
+
+    #[test]
+    fn markdown_to_html_bold() {
+        let html = markdown_to_html("**bold**");
+        assert!(html.contains("<strong>bold</strong>"), "expected <strong>: {html}");
+    }
+
+    #[test]
+    fn markdown_to_html_unordered_list() {
+        let html = markdown_to_html("- item one\n- item two");
+        assert!(html.contains("<ul>"), "expected <ul>: {html}");
+        assert!(html.contains("<li>item one</li>"), "expected list item: {html}");
+    }
+
+    #[test]
+    fn markdown_to_html_inline_code() {
+        let html = markdown_to_html("`inline_code()`");
+        assert!(html.contains("<code>inline_code()</code>"), "expected <code>: {html}");
+    }
+
+    #[test]
+    fn markdown_to_html_code_block() {
+        let html = markdown_to_html("```rust\nfn main() {}\n```");
+        assert!(html.contains("<pre>"), "expected <pre>: {html}");
+        assert!(html.contains("<code"), "expected <code> inside pre: {html}");
+        assert!(html.contains("fn main() {}"), "expected code content: {html}");
+    }
+
+    #[test]
+    fn markdown_to_html_plain_text_passthrough() {
+        let html = markdown_to_html("Hello, world!");
+        assert!(html.contains("Hello, world!"), "expected plain text passthrough: {html}");
+    }
+
+    // -- bot_context_is_clone -----------------------------------------------
+
     #[test]
     fn bot_context_is_clone() {
         // BotContext must be Clone for the Matrix event handler injection.