dg/predictable agent versioning - #13

We need to revisit the cert based flow to make this work. But for now disable the check so that we can onboard a host with a token based flow.

VERSION now defaults to git describe --dirty --tags --match='v*'
(matching kaapi's Makefile philosophy) instead of manually-bumped
MAJOR.MINOR.BUILD_NUMBER, so byohctl's baked-in version is tied to the
exact commit it was built from with no manual bump step. VERSION remains
overridable for local testing via make build VERSION=v9.9.9-test.

The hardcoded ByohAgentDebPackageURL constant required a manual
hand-edit every time the agent bundle changed. Replace it with a
ByohAgentBundleURL() function that composes the OCI repo path with
byohctl's own baked-in version.GetVersion(), so byohctl and its
expected agent bundle are always tied to the same git-describe version
with zero manual edits.

Version now comes from `git describe --dirty --tags --match='v*'`
(overridable via `BYOH_DEB_VERSION` env var) instead of an externally-
supplied `BUILD_NUMBER`. This ensures the published bundle tag matches
what byohctl bakes in at build time with no manual coordination.

Also corrects the stale "byoh-deb" echo string to "byoh-agent-deb" to
match the actual push target.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

Closes the loop: every push to main that passes CI now automatically
builds and publishes the agent deb bundle tagged with that commit's
git-describe version. This replaces the previous manual rebuild-and-
hand-edit-a-constant process.

Triggers via `workflow_run` off the existing CI workflow's success
(avoiding a duplicate test run) plus a manual `workflow_dispatch` for
dry runs.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

The agent bundle now lives at quay.io/platform9/cluster-api-provider-bringyourownhost/agent
instead of the separately-named quay.io/platform9/byoh-agent-deb. This aligns the artifact
path with the actual GitHub repository name, avoiding confusion with an unrelated flat naming scheme.

Update the publish target to match the new quay.io/platform9/cluster-api-provider-bringyourownhost/agent
path that byohctl now expects. This ensures the CI pipeline pushes to the same OCI registry location
that the agent installer references.