fix: drop charts prefix from helm-mirror destinations

Summary

Closes #451 (closed).

Remove the extra charts/ destination namespace from Helm mirror examples and integration jobs. The runtime already respects target literally, so no compatibility rewrite or automatic stripping is needed. Existing source repository paths and Helm's local charts/ dependency directory remain unchanged.

Plan

1. Publish the example chart directly below the configured target and images below target/images/.
2. Exercise prefix-free destinations in mapping, dependency write-back, and real OCI integration tests.
3. Validate the full pipeline and complete the reviewer handoff.

Acceptance criteria

• Documentation and catalog integration jobs use targets without an extra charts/ namespace.
• Chart push/pull, exact image mappings, and parent dependency write-back use the configured target directly.
• Existing source repository paths and bundled dependency directories are preserved.
• Nine local Helm mirror tests pass, including real Helm/Skopeo against a verified-TLS registry and disposable Git write-back.
• CI YAML lint, Markdown lint, rendered documentation review, final diff review, and rebase pass.
• Full current-source pipeline succeeds.

Validation

• python3 -m unittest discover -s templates/helm-mirror/bin/tests -p 'test_*.py' -v: 9 passed, no failures or skips. The local validation environment needed the OpenSSL executable installed for the existing real-TLS fixture.
• glab ci lint .gitlab-ci.yml: valid.
• Markdown lint and rendered page review: pass.
• git diff --check: pass; rebased on current main.

Current-source CI evidence

Source: 5d29f2a7ea5e5177e7cf25f828836b1599043c06.

• Full MR pipeline passes. Its parent and three component child pipelines report 47 successful jobs and three successful bridges, with no failed/skipped/unfinished jobs.
• Helm mirror child pipeline: all five jobs pass.
• Required unittest job: downloaded JUnit verifies 9 tests, zero failures/errors/skips, including the exact prefix-free mapping and real TLS OCI/Git write-back.
• Real mirror published the chart directly below the configured target. The downloaded mapping contains three images, all below ttl.sh/ci-tools-helm-mirror-3007871092/images/.
• Acceptance pulled ttl.sh/ci-tools-helm-mirror-3007871092/cert-manager:v1.18.2 and inspected the mirrored controller image.

Definition of Done

• Acceptance criteria met; no remaining implementation work.
• Diff limited to the requested destination convention and its verification.
• Required quality checks and current-source pipeline pass without bypasses.
• Final diff reviewed and rebase performed against current main.
• Documentation and generated target input reference reviewed.

The built Docusaurus Helm mirror page was downloaded from the documentation job and checked for the prefix-free example, image paths, and generated target input reference.