Make Planfile GitHub synchronization label-idempotent

[tom-sapletta-com]

Reproduction: run planfile ticket update PLF-088 --sync repeatedly. The serialized Planfile ticket gains another priority: high label on each sync although ticket priority is unchanged. This makes the manifest and GitHub issue metadata noisy.

Acceptance criteria:

1. Repeated synchronization of an unchanged ticket does not change its labels or metadata.
2. Priority projection is deduplicated or represented only by the canonical priority field.
3. Add a regression test for repeated GitHub synchronization.
4. Provide a one-time normalizer for already duplicated labels.

Reproduction: run planfile ticket update PLF-088 --sync repeatedly. The serialized Planfile ticket gains another priority: high label on each sync although ticket priority is unchanged. This makes the manifest and GitHub issue metadata noisy.

Acceptance criteria:

1. Repeated synchronization of an unchanged ticket does not change its labels or metadata.
2. Priority projection is deduplicated or represented only by the canonical priority field.
3. Add a regression test for repeated GitHub synchronization.
4. Provide a one-time normalizer for already duplicated labels.