fix(matrix): default markdown tables to bullets across Matrix clients

Matrix outbound markdown was hitting the shared `resolveMarkdownTableMode`
"code" fallback for every send because the Matrix channel plugin never
declared `defaultMarkdownTableMode` in its `messaging` block at
`extensions/matrix/src/channel.ts:452`. Tables were emitted as
`<pre><code>` fenced blocks across every Matrix client.

This change declares `defaultMarkdownTableMode: "bullets"`, matching the
Signal and WhatsApp precedent at `extensions/signal/src/shared.ts:111`
and `extensions/whatsapp/src/shared.ts:261`. The choice matches the
cross-client compatibility profile the issue filer surveyed:
Element X iOS squashes HTML `<table>` and Element X Android drops cell
text entirely (element-hq/element-x-android#1551), while bullet lists
render cleanly across every Matrix client. Operators wanting the
previous fenced-code rendering can set `channels.matrix.markdown.tables:
"code"` explicitly; clients that do render real tables can opt in with
`channels.matrix.markdown.tables: "off"` (markdown-it's `table` rule is
already enabled by default through the markdown-it default preset, so
raw markdown tables flow through to native HTML tables on that path).

Docs and the changelog entry list Matrix alongside Signal and WhatsApp
as a bullet-default channel.

Fixes #78990.

docs/concepts/markdown-formatting.md

@@ -65,7 +65,7 @@ Markdown tables are not consistently supported across chat clients. Use
 `markdown.tables` to control conversion per channel (and per account).
 
 - `code`: render tables as code blocks (default for most channels).
-- `bullets`: convert each row into bullet points (default for Signal + WhatsApp).
+- `bullets`: convert each row into bullet points (default for Matrix, Signal, and WhatsApp).
 - `off`: disable table parsing and conversion; raw table text passes through.
 
 Config keys:

extensions/matrix/src/channel.message-adapter.test.ts

@@ -238,4 +238,8 @@ describe("matrix channel message adapter", () => {
       },
     });
   });
+
+  it("declares bullets as defaultMarkdownTableMode to avoid code-block fallthrough (#78990)", () => {
+    expect(matrixPlugin.messaging?.defaultMarkdownTableMode).toBe("bullets");
+  });
 });

extensions/matrix/src/channel.ts

@@ -450,6 +450,7 @@ export const matrixPlugin: ChannelPlugin<ResolvedMatrixAccount, MatrixProbe> =
           }).map(projectMatrixConversationBinding),
       },
       messaging: {
+        defaultMarkdownTableMode: "bullets",
         targetPrefixes: ["matrix"],
         normalizeTarget: normalizeMatrixMessagingTarget,
         resolveInboundConversation: ({ to, conversationId, threadId }) =>