[SDK-4] Conflict resolution runtime — throw-by-default

[ChrisonSimtian]

Implement conflict resolution from RFC #5.

Scope

• Detect conflicting registrations (duplicate IHost for same env, duplicate middleware (phase, priority), duplicate tool wrapper name, etc.).
• Default behaviour: throw with structured error naming both plugins and the conflicting contribution.
• Override mechanism per RFC's final form.
• No silent winner-takes-all.

Depends on

SDK-3 and the per-surface extension-point tickets (SDK-6a..6f).