feat(gateway): own coordinated symbol mutation

[AntoineToussaint]

Closes the CLI-side prerequisite for codefly-dev/mind#369.

What changed

• pins Core v0.2.86 and exposes the typed analyzer-owned symbol mutation through the production Gateway;
• keeps qualified_name (analyzer selector) distinct from stable symbol_id (claim/lease/fence identity);
• returns only source-free hashes, typed failure reasons, and execution evidence to Mind;
• prepares symbol mutations through the real Codefly analyzer, retains resulting project bytes inside Codefly, and requires an exact symbol permit fence before apply;
• deletes the previous test gRPC execution mock from this path and exercises the real local Codefly source implementation.

Evidence

• GOWORK=off go test ./pkg/gateway ./pkg/engine
• real file-backed prepare performs no write, a file fence cannot authorize the symbol mutation, the exact signed symbol fence applies once, and stale anchors remain unchanged while preserving their typed recovery reason.