Maintenance: update dependencies, add tests, and set up release/OSS infrastructure

[nobuo-miura]

Summary

Perform a general maintenance pass to improve project health, expand test coverage, and prepare SecretLens for versioned releases.

Changes

Dependencies

• Migrate go-github from v72 to v89
  • Adapt to the new NewClient(opts...) (*Client, error) API
  • Include upstream security hardening, such as cross-host credential redirect protection
• Update affected indirect dependencies
• Update the Go directive to 1.26.5, which includes crypto/tls and os security fixes

Tests

• Add tests for previously untested packages:
  • internal/scanner
  • internal/baseline
  • internal/reporter/{sarif,html,slack,github}
  • internal/scanner/{cilog,envfile}
  • internal/detector/verifier
  • internal/org
  • rules

OSS and repository infrastructure

• Add weekly Dependabot updates for Go modules and GitHub Actions
• Add SECURITY.md and CONTRIBUTING.md
• Add issue and pull request templates
• Add an explicit .golangci.yml configuration and resolve the existing gocritic warning
• Add a GoReleaser configuration
• Add a release workflow triggered by v* tags

Verification

• go test -race -count=1 ./...
• go vet ./...
• go build ./...
• golangci-lint run ./...
• govulncheck ./...
• GoReleaser snapshot build succeeds for all configured targets

Follow-up

After this change is merged and CI passes, create the v0.1.0 tag to trigger the first release.