fix(core): stop duplicating the cut entry into both head and recent during compaction - #14055

What

Fixes an off-by-one in SessionCompaction.select() (packages/core/src/session/compaction.ts) that duplicated the partially-split conversation entry into both the head (summarized away) and recent (kept) outputs of auto-compaction.

Why

When the keep budget overflows mid-entry, the entry at index is split into a prefix (intended for head) and a suffix (intended for recent). But head was built with conversation.slice(0, split) where split = index + 1, so the full entry was included in head in addition to its prefix — and its suffix therefore appeared in both outputs. The summary prompt sent to the model contained duplicated content with an inflated token count, which also makes the Token.estimate(summaryPrompt) > context - summaryOutput guard more likely to abort auto-compaction entirely, leaving the session stuck on context-overflow errors.

The fix keeps separate cut points: head takes the entries before the split index plus the prefix; recent takes the suffix plus the entries after it.

Testing

Added a regression test in packages/core/test/session-compaction.test.ts that constructs four fixed-size entries with a budget that splits the third one. It fails on the previous logic (head contains the cut entry's tail) and passes with the fix. bun test (packages/core, 1325 pass) and bun run typecheck are green; the single failure in the full suite (ProjectCopy > requires force to remove a dirty git worktree) reproduces on unmodified main on this machine and is unrelated.