resume: address PR review feedback

Copilot review on #1547:

• Privacy: the non-interactive divergence/keep message no longer echoes prompt text (user content) — it identifies the conflict by session ID only, since that output can land in CI logs / captured stderr.
• Memory: TranscriptIsCleanContinuation now walks both transcripts line-by-line in place (bytes.IndexByte) instead of strings.Split-ing the whole byte slices, avoiding transient copies/allocs on large logs (and the bufio line-length cap). Adds a focused table test for its edge cases.
• Consistency: gate refreshMetadataFromRemote's spinner on CanPromptInteractively too, matching FetchBlobsByHash, so agent/CI invocations stay silent on a pty.