Set the shell-conventions rulebook to skill-only delivery

[williamthorsen]

Problem

content/guidance/rulebooks/shell-conventions.md still declares delivery: [ambient, skill]. The dual mode was a transitional fixture to exercise both delivery paths; ambient now loads shell-script guidance into unrelated sessions. (The legacy bundle this ticket was filed to retire was already removed by #733.)

Context

#732 deferred bundle retirement to this ticket; #733 (closed) removed the bundle while building the codeassembly.yaml format; #830 (closed) added consult- skill naming. Both dependencies are done. The delivery-mode flip is the only residual work.

Proposed solution

Set delivery: skill (scalar, matching the sibling authoring-guidance.md). On sync, the inlined ambient block is retracted from the host file; consult-shell-conventions skill delivery and the neutral body are unaffected. Update the real-content canary in sync.test.ts to drop its two ambient-host assertions — the skill-only mechanism stays covered by the synthetic gamma test.

Acceptance criteria

Must have

• shell-conventions delivers skill-only (delivery: skill).
• The real-content canary no longer asserts ambient delivery for shell-conventions and passes.
• Verified: no content/skills/shell-conventions/ bundle or dangling references remain, and install deploys consult-shell-conventions unaffected.