ci(hypatia-scan): fix unresolvable setup-beam version pins (#63)

## Summary

The **Hypatia Neurosymbolic Analysis (Dogfooding)** job
(`hypatia-scan.yml`, job `scan`) has been failing on **100% of runs**
across the estate, independent of PR content — red on already-merged PRs
(#58, #60) and every new PR (#62).

Root cause (inferred from a consistent ~12s fast-fail + workflow logic;
the scan step uses `--exit-zero` and the only gate is "warn but don't
fail"): the `Setup Elixir` step pinned `elixir-version: '1.19.4'` /
`otp-version: '28.3'`, which are **not present in the `setup-beam`
version index**, so the step errored immediately and failed the whole
job for everyone.

This pins to currently-published, resolvable **loose** major/minor
versions (`elixir 1.18` / `otp 27`) so patch availability cannot
re-break it.

## Changes

- `hypatia-scan.yml`: corrected `setup-beam` Elixir/OTP pins + rationale
comment.

## Test plan

- [ ] `Hypatia Neurosymbolic Analysis (Dogfooding)` goes green on this
PR (unrelated no-op change → proves the failure was environmental, not
finding-driven).
- [ ] Confirm `1.18` / `27` actually build Hypatia's scanner (`mix
escript.build`); bump deliberately if the scanner needs newer.
- [ ] Follow-up (tracked in companion issue): publish a
supported-toolchain matrix in `hyperpolymath/hypatia` + a smoke job
there so a bad pin fails *at the source*, not estate-wide.

https://claude.ai/code/session_01GTo7dz32ZgxuHXefv8BGqn

---
_Generated by [Claude
Code](https://claude.ai/code/session_01GTo7dz32ZgxuHXefv8BGqn)_

Co-authored-by: Claude <noreply@anthropic.com>

Author: hyperpolymath

.github/workflows/hypatia-scan.yml

@@ -28,8 +28,14 @@ jobs:
       - name: Setup Elixir for Hypatia scanner
         uses: erlef/setup-beam@fc68ffb90438ef2936bbb3251622353b3dcb2f93 # v1.24.0
         with:
-          elixir-version: '1.19.4'
-          otp-version: '28.3'
+          # Pinned to currently-published, setup-beam-resolvable versions.
+          # The previous pins (elixir 1.19.4 / otp 28.3) do not exist in the
+          # setup-beam index, so this step failed fast on every run and made
+          # the dogfooding job red estate-wide regardless of PR content.
+          # Major/minor (loose) so patch availability cannot re-break it.
+          # Bump deliberately to whatever Hypatia's scanner actually requires.
+          elixir-version: '1.18'
+          otp-version: '27'
 
       - name: Clone Hypatia
         run: |