refactor: replace panics with returned errors

[outofcoffee]

Replaces every production-code panic in the since CLI with returned errors and gives the "no changes since tag" failure a friendly, actionable message.

Summary

• Library helpers in changelog, semver, and vcs now return errors instead of panicking; every cobra subcommand uses RunE with SilenceUsage / SilenceErrors.
• cmd/root.go prints failures as a single Error: ... line on stderr and exits non-zero — no more Go stack traces leaking to users.
• New changelog.NoChangesError is returned when nothing remains after filtering, e.g. no eligible commits since v1.17.4 — 1 commit(s) were excluded by ignore patterns in since.yaml. A new vcs.FilterStats carries the considered/excluded counts up from processCommits.
• Tightens latent crashes: invalid ignore: regexes in since.yaml now report invalid ignore pattern … instead of panicking via regexp.MustCompile, and a repo with no tags returns no tags found in repository … instead of dereferencing a nil tag reference.

Implementation details

The refactor was split into two commits: the mechanical panic-removal pass, then the UX layer that introduces NoChangesError and the filter-stats plumbing. Keeping them apart makes the second commit easy to review on its own — it is the only part that changes behaviour beyond "no more stack traces".

FetchCommitsByTag gains a third return value (FilterStats); existing callers that don't care discard it with _. processCommits increments the Considered and Excluded counters only for non-tag commits past the skip window, so the counts reflect what the user actually intends to see in the changelog rather than the raw git log count.