[frontend-contrib S7] @netscript/plugin pointer axis (.withFrontend)

[rickylabs]

Part of #922

Scope

Additive pointer axis: .withFrontend({ export, framework }) on the builder, frontend block in the installer manifest (parse-only), CONTRIBUTION_AXES gains 'frontend'; family/major derived from the module at generate time (no duplicated handshake); PLUGIN_MANIFEST_SCHEMA_VERSION bumped additively.

Design source: RFC #890 (rfc.md §5) + .llm/runs/plan-frontend-contrib--seed/design/canonical/ (rev 3).

Acceptance

• gate: manifest schema round-trip + older-CLI ignore test
• gate: doctor prints handshake drift when module vs cache disagree