Deploy Slack across all workstation platforms (#41684)

## Summary

- Adds Slack as managed software to the Workstations fleet for
**macOS**, **Windows**, and **Linux** so it is installed on all new and
existing devices and kept up to date automatically.
- Uses **Fleet-maintained apps** (`slack/darwin`, `slack/windows`) for
macOS and Windows to ensure the latest version is always deployed.
- References the existing `slack-deb.yml` and `slack-rpm.yml` package
definitions for Linux (Debian and RPM).
- All entries include `self_service: true` and `setup_experience: true`
to install on new devices during setup and allow self-service
reinstallation.
- Mobile devices (iOS, iPadOS, Android) already have Slack configured in
both company-owned and personal mobile device fleets — no changes needed
there.

## Changes

Only `it-and-security/fleets/workstations.yml` is modified:

| Platform | Method | Entry |
|----------|--------|-------|
| macOS | `fleet_maintained_apps` | `slack/darwin` (Apple Silicon) |
| Windows | `fleet_maintained_apps` | `slack/windows` (x86) |
| Linux (Debian) | `packages` | `slack-deb.yml` |
| Linux (RPM) | `packages` | `slack-rpm.yml` |

Built for [Allen
Houchins](https://fleetdm.slack.com/archives/D0AFASNBZMW/p1773435271021419)
by [Kilo for Slack](https://kilo.ai/features/slack-integration)

---------

Co-authored-by: kiloconnect[bot] <240665456+kiloconnect[bot]@users.noreply.github.com>
Co-authored-by: Allen Houchins <32207388+allenhouchins@users.noreply.github.com>

it-and-security/fleets/workstations.yml

@@ -223,6 +223,20 @@ software:
         - Communication
       labels_include_any:
         - "RPM-based Linux hosts"
+    - path: ../lib/linux/software/slack-deb.yml # Slack for Ubuntu
+      self_service: true
+      setup_experience: true
+      categories:
+        - Communication
+      labels_include_any:
+        - "Debian-based Linux hosts"
+    - path: ../lib/linux/software/slack-rpm.yml # Slack for RedHat
+      self_service: true
+      setup_experience: true
+      categories:
+        - Communication
+      labels_include_any:
+        - "RPM-based Linux hosts"
     # Windows apps
     # Commenting out until #25407 is implemented
     # - path: ../lib/windows/software/zoom-arm.yml # Zoom for Windows (ARM)
@@ -298,6 +312,11 @@ software:
       self_service: true
       categories:
         - Communication
+    - slug: slack/darwin # Slack for macOS
+      self_service: true
+      setup_experience: true
+      categories:
+        - Communication
     - slug: okta-verify/darwin # Okta Verify for macOS
       self_service: true
       setup_experience: true
@@ -308,6 +327,13 @@ software:
       categories:
         - Productivity
     # Windows apps
+    - slug: slack/windows # Slack for Windows
+      self_service: true
+      setup_experience: true
+      categories:
+        - Communication
+      labels_include_any:
+        - "x86-based Windows hosts"
     - slug: claude/windows # Claude for Windows
       self_service: true
       labels_include_any: