fix for when filtering leads to no results

[rusq]

Followup for #695

Fix slackdump resume when stale filters remove every resume candidate.

Previously, a valid database archive could be reported as empty if -skip-stale-threads or -skip-stale-channels filtered out all generated resume
entities. The command now treats that case as a successful no-op and returns before Slack API setup.

Changes

• Changed latest(...) to return a small result struct with:
  • the generated entity list
  • whether the source had latest timestamp data
  • how many entities were skipped by stale cutoffs
• Added resume exit decision logic to distinguish:
  • truly empty archives
  • fully stale-filtered archives
  • non-empty resume selections
• Preserved explicit CLI entity overlays so user-provided entities still force work.
• Added an info log for fully stale-filtered no-op runs.
• Documented the no-op behavior in resume help.
• Expanded resume tests for stale-filtered empty results and decision behavior.

Tests

• go test ./cmd/slackdump/internal/resume