Build #104: Use .isv for visual artifact Preview

[nil4u]

Source issue

Closes #104

Summary

• Add .isv suffix discovery to the Preview Registry for visual artifacts in any file name or directory.
• Resolve .isv artifact type from the JSON artifact field while keeping the Visual Engine Schema unchanged.
• Generate and publish Decision/Plan artifacts as decision/data/decision.json.isv and plan/data/plan.json.isv, and pass the new Plan path into Build runtime inputs.
• Update runtime prompts, Vision Plan skill docs, provider docs, project plugin mirrors, web fallbacks, and test fixtures.
• Compatibility deviation: per implementation clarification, keep existing Markdown Preview and fixed-name decision-data.json / plan-data.json recognition instead of making .isv exclusive.
• Current-code deviation: no optimization-data.json.isv generator was added because type::optimization is explicitly routed to Markdown Plan and disables Visual Plan in the current runtime.

Validation

• env -u AGENTRIX_TASK_ID node --test plugin/test/submit.test.cjs plugin/test/agentrix-runtime.test.cjs plugin/test/dispatch.test.cjs plugin/test/merged.test.cjs — passed, 107 tests.
• env -u AGENTRIX_TASK_ID node --test console/api/test/visual-artifacts.test.cjs — passed, 27 tests.
• DATABASE_URL='postgresql://issue-flow:test@127.0.0.1:5432/issue_flow_test' npm run api:build — passed.
• npm run web:build — passed.
• Full API database-backed tests were not runnable because PostgreSQL was not available at 127.0.0.1:5432; non-database tests and the changed artifact suite passed.