fix(Slack): resolve Slack channels by raw ID and enumerate joined channels

send_message(target='slack:<channel_id>') failed with "Could not
resolve" because _parse_target_ref had no Slack branch — Slack's
uppercase alphanumeric IDs fell through to channel-name resolution,
which only matched by name. As a fallback, the agent would retry with
bare target='slack' and post to the home channel instead.

Three fixes:

- _parse_target_ref recognizes Slack IDs (C/G/D/U/W prefix) as
  explicit targets so the name-resolver is bypassed entirely.
- resolve_channel_name tries a case-sensitive raw-ID match before
  the existing name match, so any platform's IDs resolve cleanly.
- _build_slack now actually calls users.conversations against each
  workspace's AsyncWebClient (paginated), instead of only returning
  session-history entries. This populates the directory with public
  and private channels the bot has joined, so action='list' shows
  them and they can also be addressed by name. Errors from one
  workspace don't block others.

build_channel_directory becomes async (Slack web calls require it).
The two async-context callers in gateway/run.py are awaited; the
cron ticker thread call bridges via asyncio.run_coroutine_threadsafe.

Slack bot needs channels:read and groups:read scopes for full
enumeration; missing scopes degrade gracefully per-workspace.

addressing #15927

tools/send_message_tool.py

@@ -20,6 +20,10 @@ logger = logging.getLogger(__name__)
 
 _TELEGRAM_TOPIC_TARGET_RE = re.compile(r"^\s*(-?\d+)(?::(\d+))?\s*$")
 _FEISHU_TARGET_RE = re.compile(r"^\s*((?:oc|ou|on|chat|open)_[-A-Za-z0-9]+)(?::([-A-Za-z0-9_]+))?\s*$")
+# Slack channel/user IDs: C (public), G (private/group), D (DM), U/W (user).
+# Always uppercase alphanumeric, 9+ chars. Without this, Slack IDs fall through
+# to channel-name resolution, which only matches by name and fails.
+_SLACK_TARGET_RE = re.compile(r"^\s*([CGDUW][A-Z0-9]{8,})\s*$")
 _WEIXIN_TARGET_RE = re.compile(r"^\s*((?:wxid|gh|v\d+|wm|wb)_[A-Za-z0-9_-]+|[A-Za-z0-9._-]+@chatroom|filehelper)\s*$")
 # Discord snowflake IDs are numeric, same regex pattern as Telegram topic targets.
 _NUMERIC_TOPIC_RE = _TELEGRAM_TOPIC_TARGET_RE
@@ -318,6 +322,10 @@ def _parse_target_ref(platform_name: str, target_ref: str):
         match = _NUMERIC_TOPIC_RE.fullmatch(target_ref)
         if match:
             return match.group(1), match.group(2), True
+    if platform_name == "slack":
+        match = _SLACK_TARGET_RE.fullmatch(target_ref)
+        if match:
+            return match.group(1), None, True
     if platform_name == "weixin":
         match = _WEIXIN_TARGET_RE.fullmatch(target_ref)
         if match: