fix: skip SessionStart on compact since additionalContext is ignored

PreCompact exiting non-zero pollutes the compaction context with a
"failed" message, making Claude less likely to follow compact
instructions. Errors are now written to the state file silently and
surfaced via SessionStart:resume's additionalContext instead.

Additionally, PreCompact skips the Morph API call if a cached summary
already exists (enabling a retry path), and SessionStart:compact
detects when compact instructions were ignored and prompts the user
to run /compact again.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>