feat: add proof report output

[peg]

Summary

• Add snare prove --report for first-success proof reports covering precision canary commands, observed callbacks, status, and cleanup commands.
• Add snare prove --format json for machine-readable report output, with JSON mode implying report output.
• Update first-run docs, CLI help, and changelog to point users toward the proof report workflow.

Tests

• git diff --check
• go test ./... -count=1
• cd worker && npm ci && npm test -- --run