ci(mvp-smoke): install just before invoking the mvp-env recipe (#130)

## Summary

Adds a SHA-pinned `taiki-e/install-action` step that installs `just`
before the `MVP Smoke (Best Effort)` job runs `just --justfile Justfile
mvp-env`.

## Why

The workflow has been failing on every push with:

\`\`\`
/home/runner/work/_temp/…/sh: line 1: just: command not found
##[error]Process completed with exit code 127.
\`\`\`

The `ubuntu-latest` runner image doesn't ship `just`, and no step
installed it. Surfaced during the 2026-05-30 triage of orthogonal CI
failures alongside the dependabot deadlock (PR #128).

## SHA pin

`taiki-e/install-action@184183c2401be73c3bf42c2e61268aa5855379c1`
(v2.78.1) — matches the estate convention used across
`hypatia/.github/workflows/{ci,security,tests,build-gossamer-gui}.yml`.

## Test plan

- [ ] CI: `MVP Smoke (Best Effort)` returns success on this branch.

Author: hyperpolymath

.github/workflows/mvp-smoke.yml

@@ -37,6 +37,11 @@ jobs:
       - name: Cache Cargo
         uses: Swatinem/rust-cache@65012b490220f477f20ab979e35ae732e6de4e68 # v2
 
+      - name: Install just
+        uses: taiki-e/install-action@184183c2401be73c3bf42c2e61268aa5855379c1 # v2.78.1
+        with:
+          tool: just
+
       - name: Build release
         run: cargo build --release