fix(session): reserve output headroom when limit.input is set in isOverflow

[0xSkybreaker]

The usable() function in overflow.ts subtracted outputReserve only when using the full context window, but not when limit.input was set. This meant compaction would only trigger after consuming the full input budget, leaving zero room for the next API call's output tokens — potentially causing model calls to fail.

Root Cause

In packages/opencode/src/session/overflow.ts, the usable() function had a ternary:

return input.model.limit.input
  ? Math.max(0, input.model.limit.input - reserved)           // ← missing outputReserve!
  : Math.max(0, context - outputReserve - reserved)

The outputReserve (capped at 20K) was only subtracted in the else branch. Models with limit.input set (e.g., Claude with prompt caching) got no output headroom, meaning compaction didn't trigger until the full input budget was exhausted — leaving no room for the model to generate a response.

Fix

return input.model.limit.input
  ? Math.max(0, input.model.limit.input - outputReserve - reserved)  // ✓ fixed
  : Math.max(0, context - outputReserve - reserved)

Both branches now consistently reserve output headroom before triggering compaction.

Tests

• All 12 existing overflow tests continue to pass
• Updated 3 previously-labeled "BUG" tests to serve as regression tests verifying the fix
• The fix makes the limit.input path symmetric with the context-only path — models with identical real capacity now agree on when to compact

Related: #10634, #8089, #11086, #12621