fix(releasing): remove default OS package config (#25425)

* fix(releasing): stop installing default OS package config

Vector's Debian, RPM, Alpine, and distroless-static packages installed
config/vector.yaml at /etc/vector/vector.yaml. That config runs a
demo_logs source and prints synthesized syslog to stdout, which ends up
in journald or /var/log/ when Vector is run as a service and is a
recurring source of "is my log forwarder compromised?" confusion.

Ship config/vector.yaml as a reference example under
/usr/share/vector/examples/ instead. The /etc/vector/examples/ tree of
additional sample configs is unchanged.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

* docs(docker platform): explain that images ship no default config

Replace the line claiming the default config file 'doesn't do anything'
with a note that no default is shipped and point users at the reference
example under /usr/share/vector/examples/vector.yaml.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

* fix(rpm platform): preserve existing vector.yaml on upgrade

When upgrading from an RPM that owned /etc/vector/vector.yaml to one
that no longer does, rpm would erase the file (or save it as .rpmsave
if user-modified). Either way the active path used by the systemd unit
ends up empty after the upgrade.

Mark the path as %ghost %config(noreplace) so rpm continues to claim it
without installing anything, and any existing on-disk file is left
alone during upgrade.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

* chore(releasing): update verify-install for empty-config behavior

Fresh installs no longer drop /etc/vector/vector.yaml. Update the
publish-pipeline verification script to:

- Assert that /etc/vector/vector.yaml is absent after fresh install.
- Assert that the reference example exists at
  /usr/share/vector/examples/vector.yaml.
- Continue to verify that a user-provided /etc/vector/vector.yaml is
  preserved across a re-install.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

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Co-authored-by: Claude Opus 4.7 (1M context) <noreply@anthropic.com>