[fix] Improve error handling across operations

[lugassawan]

Summary

• Propagate ListWorktrees error in PostCreateSetup instead of silently discarding — affects add, duplicate, and restore commands
• Add max iteration cap (1000) to duplicate command suffix loop to prevent unbounded git rev-parse calls
• Surface squash-merge and stale-check errors as warnings in clean operations via new MergedResult/StaleResult types with Warnings []string
• Add Error field to CleanedItem for per-item failure visibility in RemoveCandidates
• Improve AheadBehind variable naming from cryptic a, b to using named returns ahead, behind directly

N/A

Test Plan

• make fmt — clean
• make lint — 0 issues
• make test — 1081/1081 pass
• Verify rimba duplicate with --as and auto-suffix
• Verify rimba clean --merged and rimba clean --stale show warnings when branches are skipped