## What changed
Introduce `ContextBudgeting::Elastic` to prepare captured root conversation and Node REPL evidence without the fixed per-section request caps. Elastic preparation preserves message text and candidates for later allocation while retaining storage omission notices and legacy source deduplication.
Keep `ContextBudgeting::Fixed` as the default and use it at existing call sites.
## Testing
Add tests covering elastic root message preservation and legacy source deduplication, plus REPL text, images, and omission counts in text-only and multimodal modes.
GitOrigin-RevId: ce4fae9d725694456525a413fb511166df6f5e2c