fix(opencode): bound output reservation in compaction budget - #51712

Issue for this PR

Closes #51711

Type of change

• Bug fix

What does this PR do?

usable() in src/session/overflow.ts subtracted a model's full capped output limit from
the context window instead of a bounded reservation, so a 1M model with a 512k output
capability got a 524,288 budget and compacted at 600k with ~500k of window still free.

The reservation is now bounded by the compaction buffer, limits are validated once in
normalizeLimits, and both call sites pass flags.outputTokenMax. Malformed metadata
degrades to "unknown" rather than to a small default, so a 1M model is never mistaken for
a 16k one. preserveRecentBudget is unchanged.

One semantic change worth a maintainer's call: shouldCompact returns false when the
budget is <= 0, so models with unknown limits skip predictive compaction and rely on
compactAfterOverflow. The old 0 >= 0 compacted on the first step of every turn. Happy
to flip it or gate it behind config.

How did you verify your code works?

bun test test/session/compaction-budget.test.ts test/session/compaction.test.ts
bun test test/session/processor-effect.test.ts test/session/prompt.test.ts
bun run typecheck

Covers 1M/128k/32k scaling, the 16.6k non-compaction case, malformed metadata (0,
negative, NaN, Infinity, null, string), output > context, input > context,
reasoning accounting, and retention asserted against the real preserveRecentBudget.

Screenshots / recordings

Not a UI change.

Checklist

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