chore(kits): bump PATTERNS_KIT_REF to the first Patterns release with readonly root startup scripts #649

Problem

Quickstart pins the Patterns kits at PATTERNS_KIT_REF in acq.backends/common.sh, currently a commit from the Patterns 1.17.0 line. The host-authority work under #503 depends on Patterns changes that ship in a later Patterns release: the root startup scripts marked readonly: true and the OCI engine readonly-root startup change (GSA-TTS/agentic-coding-patterns#543 and #542).

The msb read-only host-config path from #637 can only be exercised end to end with the new kit versions, so the pin has to move first.

Scope

• Bump PATTERNS_KIT_REF to the immutable commit of the first Patterns release that contains those changes.
• Update every dependent assertion and fixture (pinned-ref tests, docs/ references to the old release).
• Run the focused kit-fetch and startup tests.

Acceptance

• PATTERNS_KIT_REF resolves to the new release commit and the offline suite passes.
• A shipped kit's readonly: true startup file is staged from the host-config mount in an offline test using the real kit spec.

Trigger

Blocked on: the Patterns 1.18.0 release (release PR GSA-TTS/agentic-coding-patterns#534). Start when that release is published.

Child of #503 (follow-up). AI-assisted tracker entry (OpenCode). Human review required.