fix(manifest): apply per-package snapshot-label from config

[LudovicTOURMAN]

Summary

extractReleaserConfig did not map per-package snapshot-label, and
mergeReleaserConfig did not propagate it from the root default into
per-package configs. As a result, Java/Maven snapshot PRs fell back to
DEFAULT_SNAPSHOT_LABELS (['autorelease: snapshot']) regardless of
configuration.

This PR adds the missing mappings and a regression test.

Fixes #2779.

Reproduction (before this PR)

{
  "release-type": "maven",
  "snapshot-label": "team-a",
  "packages": {
    ".": { "snapshot-label": "team-a,team-b" }
  }
}

Expected: snapshot PRs carry the configured labels.
Observed: snapshot PRs carry only autorelease: snapshot.

Root cause

Two missing wirings in src/manifest.ts:

1. extractReleaserConfig maps label, release-label, extra-label
   but omitted snapshot-label.
2. mergeReleaserConfig propagated extraLabels from the default into
   per-package configs but not snapshotLabels, so root-level values
   never reached per-package strategies.

The schema (schemas/config.json) and the TypeScript interface already
accepted snapshot-label — only the runtime wiring was missing.

Why moving config to root does not fix it on its own

Root-level snapshot-label flows through parseConfig into
manifestOptions.snapshotLabels and is assigned to
Manifest.snapshotLabels, but that field is only used (in
manifest.ts) as a matcher when scanning existing open PRs. It does
not flow back to the strategy that labels new snapshot PRs. Both fixes
are required.

Changes

• src/manifest.ts — map snapshot-label in extractReleaserConfig;
  propagate snapshotLabels in mergeReleaserConfig.
• test/fixtures/manifest/config/snapshot-labels.json — new fixture
  mirroring extra-labels.json.
• test/manifest.ts — new test covering both the root-level matcher
  and the per-package merge/override.

Compatibility

• No API or schema change. The TS type was already in place.
• Configs that did not declare snapshot-label are unaffected.
• Configs that declared it expecting the (buggy) no-op behavior now
  get the documented behavior.

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