feat(#1422): add insert-image container-probe regression gate

[davidwkeith]

Part of #1422 — does not close it. Scaffolding only, per the issue's own triage note that this fix spans two repos and isn't safe for a single autonomous pass; see the paired sidecar PR and scope note below.

Summary

• The Phase 10.1 Task 11: real-signed App Store container smoke #81 smoke matrix's image-drop row had no non-GUI substitute: a human re-run found insert-image silently failing the same way Edit-overlay text edits are ephemeral: apply-edit writes only the container clone, never the canonical Source/ repo — silent data loss on window close #718/Edit-overlay text edits still never reach canonical Source/ — #718 regression survives #737 #1066 did for text edits — guest apply succeeds, nothing persists to host Source/.
• Adds an insert-image subcommand to anglesite-container-probe (Sources/AnglesiteContainerProbe/main.swift), alongside the existing apply-edit case-8 gate, and wires it into scripts/run-container-probe.sh.
• Asserts the reply's commit is non-nil AND that the host public/images/… asset the patched <img>'s src references actually exists on disk — the check apply-edit has no equivalent of, since text edits carry no binary asset.
• Documents the new gate in docs/qa/app-store-container-smoke-test.md alongside the existing case-8 write-up.

Paired PR check

• This change is self-contained to Anglesite/Anglesite.
• This change needs a paired PR in Anglesite/anglesite-skills (MCP sidecar server). Link: Anglesite/anglesite-skills#441.

Test plan

• swift test --package-path . — 460 tests, all passing.
• swift build --package-path . --product anglesite-container-probe — builds clean.
• xcodebuild -project Anglesite.xcodeproj -scheme Anglesite -configuration Debug build — not run; this change touches only the standalone probe executable and a doc/script, not the app target.
• Manual smoke: N/A — this is itself the no-GUI substitute for the manual smoke row.

Scope note

This is the app-side half of the "add app-side probe gate scaffolding" scope for #1422: the gate exists and compiles, but cannot pass yet. It needs anglesite-skills#441 tagged/released and this app's vendored container image bumped to consume it — until then, running scripts/run-container-probe.sh insert-image against the current container image is expected to reproduce the FAIL this issue reports (missing public/images/ asset on host). #1422's other, unconfirmed defect — why the guest reply's commit can come back nil for insert-image in the first place — needs a real-hardware repro with guest debug-pane capture and is not addressed here.

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[davidwkeith]

Reviewed runInsertImage/extractFirstImgSrc and the run-container-probe.sh/doc wiring. This mirrors runApplyEdit's existing shape closely (same router wiring, same boot/state-guard pattern, same FAIL-message style pointing at the specific #1422 defect each guard catches), and correctly reuses the pre-existing InsertImageEditBuilder rather than reinventing message construction. scripts/run-container-probe.sh isn't referenced by any .github/workflows/* job, so this gate being unable to PASS yet (documented up front, pending anglesite-skills#441 + an image bump) can't turn CI red — it's purely an author-run gate, as intended.

Two minor, non-blocking notes:

• Data(base64Encoded: onePixelPNGBase64)! (main.swift, the 1x1 PNG fixture) force-unwraps a hardcoded literal. Low risk since the string is a fixed compile-time constant, but if that literal is ever hand-edited a typo turns into a crash rather than a clear "FAIL: bad fixture" message, which is otherwise this file's consistent failure-reporting style.
• mergeable_state is currently dirty even though base.sha matches current main HEAD (3bb57cf6) and the diff (probe file, doc, script) doesn't obviously overlap anything else in flight — worth a git merge locally to confirm this isn't a real conflict before merging, since GitHub sometimes just hasn't finished recomputing mergeability on a brand-new PR.

No functional issues found in the new assertions themselves — the '<img src="...">' regex extraction is appropriately scoped (documented as fixture-only, not a general parser) and the three-stage guard order (commit → <img> present → referenced asset present) matches the PR description's stated intent.

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