feat: expose on_existing_tag input in release pipeline (skip mode for re-runs)

[gandalf-at-lerian]

Context

When a pipeline re-run (attempt #2+) hits the tag existence check step before build/push, the current default behavior is on_existing_tag: fail, which aborts immediately — even when the intent is just to re-emit GitOps artifacts so the cluster can be updated.

Problem

The go-pipeline-template workflow (build.yml) supports an on_existing_tag input, but the caller-facing wrapper (release.yml) does not expose or forward this input. As a result, consumers cannot set skip mode on re-runs.

Proposed Change

1. Add on_existing_tag as an optional input in the caller-facing workflow (e.g. release.yml) with default fail.
2. Forward the input through to the inner build.yml template call via with:.
3. When set to skip: treat existing tag as OK (warning only), skip build/push, and still emit GitOps artifacts so the cluster update proceeds — evaluated per component (matrix).

Behavior Contract

Value	Tag exists	Action
fail (default)	→ error, abort	current behavior preserved
skip	→ warning, continue	skip build/push, emit GitOps artifact

Acceptance Criteria

• on_existing_tag: skip is usable from the consumer repo without changes to template internals.
• Each matrix component is evaluated independently.
• GitOps artifact is emitted even when build is skipped.
• fail remains the default (no behavior change for existing callers).

Notes

• skip does not validate that the existing image matches the current commit — immutability by convention is assumed for rc/release tags.
• This is an idempotency/re-run feature, not a replacement for fail on fresh releases.

Requested by: Bedatty