docs: add user lookup guidance for @mentions in skill

Plain @name text in message bodies doesn't ping anyone in Slack.
The skill now documents the required workflow: look up user IDs
with `slackbuzz user list --json`, then use `<@userid>` format.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

Author: isaacrowntree

skills/slackbuzz-cli/SKILL.md

@@ -200,11 +200,39 @@ slackbuzz user info @alice
 | `--since <duration>` | Time filter (2h, 1d, 7d, 2w, or YYYY-MM-DD) |
 | `--limit <n>` | Max results |
 
+## @Mentioning Users in Messages
+
+**IMPORTANT**: When mentioning users in channel messages, you MUST look up their Slack user ID first and use the `<@USERID>` format. Plain `@name` text will NOT ping anyone — it renders as literal text.
+
+### Step 1: Look up the user
+
+```bash
+# Search by name (case-insensitive, matches username and display name)
+slackbuzz user list --json | jq '.[] | select(.real_name | test("michelle"; "i")) | {name, real_name, id}'
+
+# Or list all users and grep
+slackbuzz user list --json | jq '.[] | {name, real_name, id}'
+```
+
+### Step 2: Use `<@USERID>` in the message
+
+```bash
+# Correct — will actually ping the user
+slackbuzz message send '#channel' '<@U01LM9D2MPU> <@U02P3QC5H24> please review this'
+
+# WRONG — renders as plain text, no ping
+slackbuzz message send '#channel' '@michelle @herman please review this'
+```
+
+The `<@USERID>` format is Slack's native mention syntax. Always resolve names to IDs before composing messages that need to notify someone.
+
+**When the user says "message @someone"**: First run `slackbuzz user list --json | jq` to find the matching user ID, then use `<@ID>` in the message body.
+
 ## Key Behaviors
 
-- **DM auto-detection**: `@user`, `U...` IDs, and bare names auto-resolve to DM channels
+- **DM auto-detection**: `@user`, `U...` IDs, and bare names auto-resolve to DM channels (for the channel/target argument, not message body)
+- **@mentions in message body**: Must use `<@USERID>` format — look up IDs with `slackbuzz user list --json` first
 - **Case-insensitive resolution**: User lookup matches display names and usernames regardless of case
 - **Dual tokens**: Bot token for channel ops, user token for search/DMs/status
 - **Deeplinks**: Output includes clickable Slack deeplinks
 - **Cross-tool**: Digest combines Slack, ClickUp, and GitHub activity
-- **@mentions**: Works naturally in messages — `slackbuzz message send @alice "text"`