ci: consolidate erlef/setup-beam pins on v1.24.0 (3 workflows) (#289)

## Summary

Standards had two distinct `erlef/setup-beam` SHAs and one wrong version
comment across canonical workflows. Consolidated to
`fc68ffb90438ef2936bbb3251622353b3dcb2f93` (v1.24.0, 2026-03-30) with
correct annotations.

## Before / after

| File | Before | After |
|---|---|---|
| `hypatia-scan.yml` | `fc68ffb...` ` # v1.18.2` (comment wrong; SHA is
actually v1.24.0) | `fc68ffb...` ` # v1.24.0` |
| `hypatia-scan-reusable.yml` | same as above | same fix |
| `elixir-ci-reusable.yml` | `5304e04...` ` # v1.18.2` (stale pin from
2024-09-25) | `fc68ffb...` ` # v1.24.0` |

Both SHAs verified real via `gh api
repos/erlef/setup-beam/commits/<sha>`. Fragmentation likely from
incremental updates that didn't sweep all three files together.

## Why this matters

`elixir-ci-reusable.yml` is the load-bearing change: ~12 estate repos
call it as a wrapper. This PR propagates the v1.18.2 → v1.24.0 upgrade
across the fleet through that single dependency.

## Provenance

Companion to:
- `hyperpolymath/rsr-template-repo#81` (template fakes fixed, merged)
- `hyperpolymath/odds-and-sods-package-manager#39` (downstream)
- `hyperpolymath/proven#93` (downstream, merged)
- `hyperpolymath/proven-servers#19` (downstream)

All fan-out from the SNIFS CI gate work in `hyperpolymath/snifs#30`.

## Test plan

- [ ] All 3 setup-beam pins resolve to `fc68ffb...` with comment `#
v1.24.0`
- [ ] `gh api
repos/erlef/setup-beam/commits/fc68ffb90438ef2936bbb3251622353b3dcb2f93`
returns 200
- [ ] `hypatia-scan` workflow runs green on this PR
- [ ] No downstream repos using elixir-ci-reusable break under v1.24.0
(Elixir/OTP version inputs unchanged)

Author: hyperpolymath

.github/workflows/elixir-ci-reusable.yml

@@ -118,7 +118,7 @@ jobs:
           ref: ${{ github.ref }}
 
       - name: Set up BEAM (OTP + Elixir)
-        uses: erlef/setup-beam@5304e04ea2b355f03681464e683d92e3b2f18451 # v1.18.2
+        uses: erlef/setup-beam@fc68ffb90438ef2936bbb3251622353b3dcb2f93 # v1.24.0
         with:
           otp-version: ${{ inputs.otp-version }}
           elixir-version: ${{ inputs.elixir-version }}

.github/workflows/hypatia-scan-reusable.yml

@@ -94,7 +94,7 @@ jobs:
           fetch-depth: 0  # Full history for better pattern analysis
 
       - name: Setup Elixir for Hypatia scanner
-        uses: erlef/setup-beam@fc68ffb90438ef2936bbb3251622353b3dcb2f93 # v1.18.2
+        uses: erlef/setup-beam@fc68ffb90438ef2936bbb3251622353b3dcb2f93 # v1.24.0
         with:
           elixir-version: '1.18'
           otp-version: '27'

.github/workflows/hypatia-scan.yml

@@ -51,7 +51,7 @@ jobs:
           fetch-depth: 0  # Full history for better pattern analysis
 
       - name: Setup Elixir for Hypatia scanner
-        uses: erlef/setup-beam@fc68ffb90438ef2936bbb3251622353b3dcb2f93 # v1.18.2
+        uses: erlef/setup-beam@fc68ffb90438ef2936bbb3251622353b3dcb2f93 # v1.24.0
         with:
           elixir-version: '1.18'
           otp-version: '27'