[Refactor] Centralize suggestion launch planning

[roomote-roomote]

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What changed

• Added a typed preparation boundary that resolves suggestion delivery mode, candidate repositories, canonical targets, environment selection, persistence eligibility, and per-suggestion repository IDs.
• Simplified the submission endpoint to persist and deliver a single prepared launch plan instead of re-deriving launch policy from several booleans and optional values.

Why this change was made

Current-thread routing and organization-wide suggestion support had made the submission endpoint a second source of truth for launch policy. Keeping those decisions in one preparation result reduces the chance that future suggestion modes apply repository or routing rules inconsistently.

Impact

There is no intended user-facing behavior change. Suggestion submission, persistence, and provider delivery retain their existing contracts while the launch-planning path becomes easier to extend and review.

[roomote-community]

No code issues found. See task

_{Reviewed 016f726}