refactor: isolate legacy product intelligence runtime

[eamirian]

Outcome

Starts ACE 0.8B runtime-boundary realignment by removing the embedded ACE-product intelligence branch from default Core startup.

The historical competitor/community/release/whitespace sentinel engines remain import-compatible behind the explicit ENABLE_LEGACY_PRODUCT_INTELLIGENCE=true migration switch. The default 0.8 runtime now expects Domain Packs plus separately authorized connectors.

Accepted 0.8 input

PR #126 / AM4 was explicitly compatibility-reviewed and merged first. The combined 0.8A + AM4 tree passed 157 roadmap, boundary, exact-eleven, and AM1–AM4 tests. This PR builds from main@bb7f4ba.

Changes

• adds one frozen legacy-engine compatibility manifest and registration seam
• disables implicit registration of four product-specific engines
• keeps the domain-neutral knowledge-graph community summarizer enabled
• documents the opt-in switch and rollback
• adds machine-checked default/opt-in/import-boundary tests
• records the remaining 0.8B convergence work; this PR does not close 0.8B or issue Roadmap: ace-core 0.8.0 — Intelligence OS Realignment #40

Evidence

• focused runtime/boundary/config/roadmap and legacy engine matrix: 120 passed
• Ruff check: passed
• Ruff format check: passed
• git diff --check: passed

Compatibility and rollback

No stored record, schema, API, Domain Pack, connector, provider, or thin MCP tool changes. Revert this commit to restore implicit registration, or use the explicit compatibility flag during a bounded migration. The flag grants no source, model, or action authority.

Checklist

• No private data, credentials, proprietary fixtures, or private-extension code
• No domain code copied into Core
• Exact eleven-tool public MCP surface unchanged
• Documentation and rollback updated
• Focused and compatibility tests passed

Part of #40. This is 0.8B1, not 0.8 release closeout.