[openapi-mcp S9] Activation surfaces: instructions, AGENTS.md line, cross-references, migration fixture

[rickylabs]

Part of #1126

Scope

Activation surfaces (#1071/#1072 lineage): one sentence in the server initialize instructions naming the curl moment; one behavioural line in the scaffolded app-scoped AGENTS.md; endpoint-shaped findings in get_recent_errors/doctor output cross-reference get_operation_schema; and the S-18 migration fixture — .mcp.json entries are exact-version-pinned, so an existing project stays on the old 14-tool server until agent init re-run + host restart; the fixture starts from prior-release host files and proves the new tools appear after the documented path. "Zero install" is claimed for new scaffolds only.

Design source: RFC #1123 (rfc.md §2.7) + design/canonical/05-activation.md (rev 2).

Acceptance

• gate: byte fixtures — instructions sentence present; AGENTS.md template line present; failure-path pointer present
• gate: migration fixture from prior-release .mcp.json proves the tool triad appears post re-init