fix(context): use input window for usage percent

[lin-hongkuan]

Issue for this PR

Closes #32119

Type of change

• Bug fix
• New feature
• Refactor / code improvement
• Documentation

What does this PR do?

Context usage displays were dividing by model.limit.context. For split-window models, limit.context includes both input and output windows, while prompt-fill pressure should use the input window when model.limit.input is present.

This PR adds a TUI helper that prefers a positive limit.input and falls back to limit.context, then uses it in the prompt footer, sidebar context plugin, and subagent footer. It also makes the direct opencode run footer limit map use the same effective context limit.

How did you verify your code works?

• bun test test/util/model.test.ts from packages/tui
• bun test test/cli/run/runtime.boot.test.ts from packages/opencode
• bun run typecheck from packages/tui
• bun run typecheck from packages/opencode
• git diff --check from the repo root

Screenshots / recordings

Not included; this is display denominator logic covered by unit tests.

Checklist

• I have tested my changes locally
• I have not included unrelated changes in this PR

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