ci: use artifacts by default for test image builds

[yanksyoon]

Summary

• Use GitHub Actions artifacts instead of GHCR for default test-event image builds.
• Keep upload-image: registry as an explicit opt-in for tests requiring a remote image.
• Add configurable 30-day artifact retention.
• Provision a local registry for legacy artifact-based integration tests.
• Update action bundles, documentation, and changelog.

This addresses ISD-512 by preventing routine pull-request and test builds from creating persistent GHCR versions.

Test plan

• npm run lint
• npm run format:check
• npm run package
• YAML parsing and static workflow checks

[yanksyoon]

Review comments.

[yanksyoon]

Let's set the release date to 2026-08-08

[github-actions]

Test results for commit 058a2c7

Test coverage for 058a2c7

Name           Stmts   Miss Branch BrPart  Cover   Missing
----------------------------------------------------------
src/charm.py       8      8      0      0     0%   8-25
----------------------------------------------------------
TOTAL              8      8      0      0     0%

Static code analysis report

Run started:2026-08-28 02:41:25.120930+00:00

Test results:
  No issues identified.

Code scanned:
  Total lines of code: 56
  Total lines skipped (#nosec): 0
  Total potential issues skipped due to specifically being disabled (e.g., #nosec BXXX): 0

Run metrics:
  Total issues (by severity):
  	Undefined: 0
  	Low: 0
  	Medium: 0
  	High: 0
  Total issues (by confidence):
  	Undefined: 0
  	Low: 0
  	Medium: 0
  	High: 0
Files skipped (0):

[github-actions]

Test results for commit 058a2c7

Test coverage for 058a2c7

Name           Stmts   Miss Branch BrPart  Cover   Missing
----------------------------------------------------------
src/charm.py       8      8      0      0     0%   8-25
----------------------------------------------------------
TOTAL              8      8      0      0     0%

Static code analysis report

Run started:2026-08-28 02:44:28.227256+00:00

Test results:
  No issues identified.

Code scanned:
  Total lines of code: 56
  Total lines skipped (#nosec): 0
  Total potential issues skipped due to specifically being disabled (e.g., #nosec BXXX): 0

Run metrics:
  Total issues (by severity):
  	Undefined: 0
  	Low: 0
  	Medium: 0
  	High: 0
  Total issues (by confidence):
  	Undefined: 0
  	Low: 0
  	Medium: 0
  	High: 0
Files skipped (0):

[github-actions]

Test results for commit 058a2c7

Test coverage for 058a2c7

Name           Stmts   Miss Branch BrPart  Cover   Missing
----------------------------------------------------------
src/charm.py       8      8      0      0     0%   8-25
----------------------------------------------------------
TOTAL              8      8      0      0     0%

Static code analysis report

Run started:2026-08-28 02:41:20.200265+00:00

Test results:
  No issues identified.

Code scanned:
  Total lines of code: 56
  Total lines skipped (#nosec): 0
  Total potential issues skipped due to specifically being disabled (e.g., #nosec BXXX): 0

Run metrics:
  Total issues (by severity):
  	Undefined: 0
  	Low: 0
  	Medium: 0
  	High: 0
  Total issues (by confidence):
  	Undefined: 0
  	Low: 0
  	Medium: 0
  	High: 0
Files skipped (0):