M5.6: Wire `forkguard report` subcommand

[bjcorder]

Labels: type:feature, area:cli, priority:critical,
status:blocked, milestone:M5, phase:9

Summary

Wire the forkguard report subcommand to render scan results in the
requested format.

Acceptance criteria

• forkguard report --scan <id> --format {json,sarif,table} [--output <path>]
• Default format is table
• When --output is omitted, writes to stdout
• When --output is provided, writes to file
• Helpful error if scan ID doesn't exist

Dependencies

Depends on: #25, #26, #27

Verification

./forkguard scan --fork test/fork-vulnerable
./forkguard report --scan 1 --format json --output /tmp/r.json
jq '.findings | length' /tmp/r.json
./forkguard report --scan 1 --format sarif --output /tmp/r.sarif
jq -r '.version' /tmp/r.sarif  # outputs "2.1.0"
./forkguard report --scan 1 --format table

This completes Milestone M5.

---

Milestone M6: Demo, Release Engineering & v0.1

---