reap release candidate tags within each component's namespace

[joshua-temple]

Problem / why

Release-candidate tag reaping enumerates tags by the global tag_prefix
(internal/release/release.go). For a multi-component manifest with per-component tag
namespaces, reaping must be confined to the component's prefix so it does not delete another
component's rc tags.

Scope (firm intent, to be refined after P1)

• Drive cleanupRCTags by the component's tag_prefix, replacing the global ListTags /
  listTags enumeration with a per-component-namespace query.

Files / areas touched

• internal/release/release.go.

Tests and coverage required

• Unit / e2e: rc-tag reaping confined to one component's namespace; another component's rc tags
  are untouched.
• A single-component regression test that rc-tag reaping is unchanged.

Acceptance criteria

• rc-tag reaping never crosses a component's tag namespace.
• Single-component reaping is unchanged.

Dependencies

Blocked by the independent promotion milestone. Can land alongside R1 / R2.

[joshua-temple]

Implemented and merged via #521.