fix(core): prefer runtime bootc tag over build-time image-info

[castrojo]

Closes projectbluefin/common#820

get_system_info() previously reported the image tag baked into /usr/share/ublue-os/image-info.json at build time. After a bootc switch, that tag is stale and bctl status shows the old tag.

Changes:

• Parse the tag from bootc status --json and use it when available.
• Fall back to image-info.json when bootc is missing or the runtime ref has no tag.
• Add tests covering standard refs, registry:port refs, digest refs, bare refs, and the bootc-status precedence.

Validation:

• pytest — 111 passed
• ruff check — clean
• mypy — clean

[castrojo]

Vanguard Lab Strike Report — bluefinctl#23

Field	Value
Workflow	bluefinctl-pr-23-test-fjh7d
Phase	Succeeded
Duration	20s
PR head SHA	5c3595648a3f
Suites / tests	full pytest suite

Results

111/111 tests passed (0 failures, 0 skipped)

Workflow output confirms the suite was cloned from branch fix/image-tag-from-bootc-status at projectbluefin/bluefinctl and the venv-based pytest run completed green.

Evidence collected

• argo_get_workflow — phase Succeeded, node summary 1/1
• k8s_pods_log — test run log captured (111 passed in 2.36s)

Merge eligibility

• Lab workflow successful
• GitHub Actions test (3.13) still pending
• Required approvals / merge rules not confirmed from this context

Lab test is green; waiting on GitHub Actions and review before merge queue.

[hanthor]

Approve. The runtime-tag precedence is correct and the fallback chain is sound. _tag_from_image_ref handles the tricky cases properly — registry:port refs (rsplit on the last colon, then the '/' guard rejects 5000/bluefin), digest refs (@ guard), and bare refs — and the booted.image.image.image nesting matches bootc's status schema. Tests cover the precedence and both fallback paths. CI green (test 3.13). LGTM.