Derive context-window guard thresholds from the effective model window, keeping 10% hard-min and 20% warning ratios with 4k/8k floors.
Stop the embedded runner from forcing old fixed guard overrides so runtime admission uses the dynamic resolver.
Validation:
- CI run 25151866833 passed, including build-artifacts and checks-node-channels.
- Parity gate 25151866868 passed.
- Testbox pnpm test:channels passed: 54 files / 433 tests.
Fixes#42999.
Prepared head SHA: 9c80383639