ADR 008: Define baseline mode for CI adoption

[Krishan27]

Required CLI flows

flaglint audit ./src --write-baseline flaglint-baseline.json
flaglint validate ./src --baseline flaglint-baseline.json --fail-on-new

Acceptance criteria

• ADR merged
• Baseline file format documented and versioned
• Behavior for unchanged/new/resolved debt documented
• Invalid baseline behavior and exit code documented
• File-move edge case documented (path change = new fingerprint, re-run --write-baseline after refactor)

Labels: v1.0, adr, ci | Depends on: ADR 007

[Krishan27]

Duplicate of #165 — closed by ADR 008 in PR #176.