bug(conventional-commits): PR commit bumps override (downgrade) merge commit bump

**Describe the bug**

When a squash-merged PR has a breaking change indicator in its title (e.g. `feat!:`), the conventional-commits plugin correctly parses the merge commit as `major`. However, the `parseCommit` hook then fetches the individual pre-squash PR commits and unconditionally overwrites the bump with the highest bump found among those commits — which may be lower (e.g. `minor` from `feat:` commits).

This means the merge commit's breaking change signal is silently lost.

**To Reproduce**

1. Configure auto with the `conventional-commits` plugin
2. Create a PR with individual commits like `feat: add feature` and `chore: update config`
3. Set the PR title to `feat!: breaking change` (note the `!`)
4. Squash-merge the PR (GitHub uses the PR title as the squash commit message)
5. Run `auto shipit -vv`
6. Observe the log output: `Found "minor" from conventional commit message: feat!: breaking change`
7. The release is `minor` instead of `major`

**Expected behavior**

The release should be `major` because the squash merge commit message is `feat!:`, which the plugin's own `breakingHeaderPattern` correctly matches.

**Screenshots**

Build log showing the bug in practice:
```
Found "minor" from conventional commit message: feat!: AB-4116 - add shared `@xxx/auto-config` package
```

**Environment information:**

```txt
auto version: 11.3.6
node version: v22.x
conventional-commits plugin version: 11.3.6
```

**Additional context**

The root cause is in `plugins/conventional-commits/src/index.ts`, in the `parseCommit` hook:

```typescript
let bump = await getBump(message); // correctly returns "major" for "feat!:"

if (commit.pullRequest && !prHasSemverLabel) {
const prBumps = /* bumps from individual PR commits */;
// Unconditionally overwrites bump — can downgrade major → minor
if (prBumps.includes(SEMVER.major)) {
bump = SEMVER.major;
} else if (prBumps.includes(SEMVER.minor)) {
bump = SEMVER.minor; // ← overwrites "major" with "minor"
}
}
```

The PR commit bumps should only be able to *upgrade* the release type, never downgrade it. The fix is to include the merge commit's own bump in the comparison array.

Possibly related: #2280 (feature request about using PR titles as the conventional commit source)