enhancement: add --baseline flag to compare findings against a saved snapshot

[depmedicdev-byte]

What

Add ci-doctor --baseline=baseline.json so a CI run only fails on
findings that are new relative to a checked-in snapshot. This makes
adoption viable on legacy repos that have hundreds of pre-existing
warnings.

Why

Every linter that landed in a real org needed this. Without it, the
first run is noise; with it, teams can ratchet the bar over time.

Sketch

1. ci-doctor --json --baseline-out baseline.json writes the current
   set of {ruleId, file, line} tuples to disk.
2. ci-doctor --baseline baseline.json filters out any finding whose
   tuple is in the baseline before printing.
3. --baseline should ignore line drift (match on
   {ruleId, file, normalized-message} to allow line shifts).

Acceptance

• Two new flags work end-to-end
• Test in test/baseline.test.js covering: baseline empty, baseline
  exact, baseline with line drift, baseline with new finding
• README "Adopting on a legacy repo" section added

Estimate

~3-4 hours.