fix(image): preserve active and infrastructure aliases

[stephenlclarke]

Type of Change

• Bug fix
• New feature
• Breaking change
• Documentation update

Motivation and Context

Closes #2030.

Use OCI digests as active-image identity, normalize configured infrastructure aliases, and persist the fetched builder reference. This prevents image prune --all from deleting a normalized image used by an active builder and avoids unnecessary builder recreation.

This supersedes the earlier closed #2031, rebased onto current main.

Testing

• Tested locally
• Added/updated tests
• Added/updated docs

Negative control with the former reference-only decisions: three alias cases and the digest identity regression failed.

• Focused alias/prune regressions: 14 tests passed
• Full non-integration suite: 758 tests passed
• End-to-end builder-prune regression added for integration CI
• make fmt, make check, and git diff --check: passed
• Commit signature: verified

[stephenlclarke]

Refresh 2026-08-26: rebased the single signed fix onto current main (d96dfa3). New head: d2753b6. Focused UtilityTests and ImagePruneTests passed 14/14, and git diff --check passed.

[stephenlclarke]

Rebased cleanly onto current Apple main (d925dab865cf). The updated head is signed and remains one commit over the base: fcd00e85f43cedc7a248edf38958484e882f1135.

Exact-head focused verification: swift test --jobs 4 --filter 'ImagePruneTests|UtilityTests' — 14 focused tests passed; git diff --check passed and the worktree is clean. The broader test evidence already recorded in this PR remains unchanged; this update only incorporates the newer Apple base.

@codex review

[stephenlclarke]

Refresh 2026-09-05: rebased the single signed fix onto current Apple main (eee7ad097079). The new head is 7df71fcb0e11a0bf9422e05cda35008438e5bf4b; swift test --jobs 4 --filter 'ImagePruneTests|UtilityTests' passed 14/14 and git diff --check passed. The PR is mergeable; fork-originated CI still requires Apple approval.