tip: use effective input limit for context % (#51250)

[kvnloo]

Tip leaf

Upstream tip (not touching origin): anomalyco#51250

What

Use the model's effective input limit (when present) for context % display instead of always using the context window size.

Proof

fail-then-pass on packages/tui/test/util/model.test.ts -t contextLimit

• NEGATIVE (test-only): 0 pass / 1 fail
• GREEN (full tip): 5 pass / 0 fail
• RESTORE (test-only): 0 pass / 1 fail

Notes

CLEAN apply on fork dev. github_writes_origin=0. Codex promote parked.