feat: add findings summary command (per-resource rollup)

[forsythfamous]

Closes #16

Adds cloudcost findings summary, a rolled-up view of findings.json grouped by resource_id, for deciding which resource to fix first when one resource trips several checks (e.g. stopped-not-deallocated and untagged).

For each resource it shows the number of findings, the policies that fired, and the combined estimated impact, sorted by impact (highest first), followed by a total.

┃ Resource    ┃ Findings ┃ Policies                          ┃ Combined impact ┃
│ /sub/disk-b │        1 │ unattached_disks                  │         $250.50 │
│ /sub/vm-a   │        2 │ stopped_not_deallocated_vms,      │         $100.00 │
│             │          │ untagged_resources                │                 │
Total combined impact: $350.50

• --min-findings N shows only resources with at least N findings (--min-findings 2 = resources hit by multiple checks)
• --input accepts a findings file path, matching findings notify
• Findings without a resource_id (e.g. the GitHub Actions check) are skipped, since there is nothing to group them on
• findings.json itself is unchanged; each finding stays independent

Testing

• New tests/test_findings_summary.py: grouping and sort order, the --min-findings filter, and a missing input file
• Full suite: 73 passed
• README usage section updated