docs(matrix-mcp): add send_file, download_file, unread guard to tools/matrix.md

The human-readable matrix MCP reference was missing three behaviours
documented only in the MCP server instructions string:

- send_file / download_file tools (issues 1829, 1830)
- [file:]/[image:]/[audio:]/[video:] attachment markers in read_room (issue 1830)
- unread guard: send_message, send_dm, send_file, send_reply all
  reject if the room has unread messages (issue 1828)
- send_redact (omitted from the tool list entirely)

Add all four to docs/tools/matrix.md to match the MCP server
instructions added in the previous commit.

docs/tools/matrix.md

@@ -8,17 +8,35 @@ a second stdio MCP server. Tools land as `mcp__matrix__<name>`:
 
 ### Messaging
 
+**Unread guard**: `send_message`, `send_dm`, `send_file`, and
+`send_reply` are all rejected if the room has unread messages — call
+`read_room` then `mark_read` on the latest event before sending to a
+room you haven't read yet.
+
 - `send_message(room, body)` — send a markdown message; `room`
   accepts `!id:server` or `#alias:server`, daemon resolves either
-- `send_dm(user_id, body)` — send a direct message to a matrix user
+- `send_dm(user_id, body)` — open (or reuse) the DM room with
+  `user_id` and post `body` to it
 - `send_reply(room, event_id, body)` — threaded reply to a specific
   event
 - `send_reaction(room, event_id, key)` — react to a message with an
   emoji key
+- `send_file(room, path, caption?)` — upload a local file and post it
+  as a room attachment; MIME type is inferred from the file extension,
+  50 MiB cap; optional `caption` is sent as a follow-up text message
+- `send_redact(room, event_id, reason?)` — redact (delete) a specific
+  event; works on your own events, redacting others' requires moderator
+  power level
 
 ### Reading
 
-- `read_room(room, limit?)` — recent timeline events
+- `read_room(room, limit?)` — recent timeline events; media attachments
+  appear inline as `[file: name]`, `[image: name]`, `[audio: name]`,
+  or `[video: name]` markers — pass the event id to `download_file`
+  to retrieve the attachment
+- `download_file(room, event_id, dest_path?)` — download a media
+  attachment from a message event to a local file and return its path;
+  pair with `read_room` which surfaces attachments as the markers above
 - `list_rooms()` — enumerate joined rooms
   (`{ id, canonical_alias, name, member_count }` per room)
 - `list_room_members(room)` — members of a room