fix(pr-pipeline): hold lane PRs on marketplace sources and cost paths under native auto-merge #9449

Refs: #9276

Problem

Under merge.lane-prs: native (#9448), a lanes App pull request is held only by ci-status and enable-auto-merge.mjs. Both read HOLD_PATH_FLOOR in .github/scripts/pr-merge/merge-holds.mjs, which lacks two path sets that the pr-merge gate scores and holds. The gate's own hold was already a slow backstop, because it escalates only after a refusal has stood for merge.risk-escalation-minutes. Under native that backstop is gone. Found by the security review of #9448; these gaps predate it.

• .claude-plugin/marketplace.json. The gate scores a marketplace entry that is added, removed, or has its source changed as HIGH (babysit_risk.py is_manifest_path and _marketplace_changes). HOLD_PATH_FLOOR covers only **/.claude-plugin/plugin.json, CODEOWNERS, .gitmodules and the forced paths. A lane PR could therefore repoint a plugin's source to an external repository with no hold. The pr-pipeline README's phrase "every plugin manifest" overstates the floor.
• Cost paths. The gate's cost-review-required reasons (babysit_risk.py COST_PATHS: plugins/multi-agent/reference/defaults.yaml, Pulumi, Terraform and Bicep files) are not hold paths. A lane PR can raise model or effort spend with no owner decision.

Done when

• ci-status and enable-auto-merge hold a lanes App PR that changes a marketplace entry's source, or adds or removes an entry, until a trusted person approves the head.
• They hold a lanes App PR that changes a cost path the same way.
• A test fails before the fix for each.