Simplify: reuse prompt-join and settings-path helpers

Quality pass (/simplify) findings:

• Export checkpoint.RedactedJoinedPrompts (was the private redactedJoinedPrompts) and call it from fsstore instead of inlining join+redact twice, so alternate backends produce identically-redacted prompt blobs through the canonical path.
• Extract settingsAbsPaths/worktreeSettingsPaths helpers in the settings package and use them from Load, loadForWorktreeRoot, and checkpointsSettingsPaths, removing the triplicated base+local path resolution.

No behavior change; full unit suite green.