feat(sign-cache): increase default concurrency and add configurability

[leodido]

Summary

Increases default signing concurrency from 5 to 20 concurrent operations to better utilize network I/O for Sigstore API calls, providing ~4x faster signing for typical workloads.

Fixes https://linear.app/ona-team/issue/CLC-2084/improve-the-performance-of-attestation-signing-with-configurable

Changes

• Increase default concurrency: 5 → 20 (4x improvement)
• Add --max-signing-concurrency flag: Runtime configuration (default: 20)
• Add LEEWAY_MAX_SIGNING_CONCURRENCY env var: Environment-based configuration
• Proper precedence: Env var sets default, explicit flag overrides (follows Leeway pattern)
• Automatic validation: Bounds checking (1-100 range) with warning logs
• Comprehensive tests: Env var/flag precedence and validation coverage
• Enhanced documentation: Help text with concurrency section and examples
• Code cleanup: Removed unused cmd parameter from runSignCache

Performance Impact

• Current: 100 artifacts ÷ 5 = 20 batches × 3s = ~60 seconds
• Optimized: 100 artifacts ÷ 20 = 5 batches × 3s = ~15 seconds
• Gain: ~4x faster signing for typical workloads
• Configurable: Can be tuned up to 100 for high-throughput scenarios

Implementation Details

Follows existing Leeway patterns:

• Environment variable handling uses cmd.Flags().Changed() pattern (same as in-flight-checksums)
• Naming convention follows LEEWAY_* pattern
• Structured logging with log.WithField()
• Test patterns match existing test suites

Testing

All tests pass:

• ✅ TestMaxSigningConcurrencyEnvironmentVariable - 8 test cases for env var/flag precedence
• ✅ TestMaxSigningConcurrencyValidation - 6 test cases for bounds validation
• ✅ All existing sign-cache tests continue to pass
• ✅ Full test suite: go test ./... -short

Examples

# Use default (20)
leeway plumbing sign-cache --from-manifest artifacts.txt

# Override with flag
leeway plumbing sign-cache --from-manifest artifacts.txt --max-signing-concurrency 30

# Override with env var
LEEWAY_MAX_SIGNING_CONCURRENCY=30 leeway plumbing sign-cache --from-manifest artifacts.txt

# Flag takes precedence over env var
LEEWAY_MAX_SIGNING_CONCURRENCY=10 leeway plumbing sign-cache --from-manifest artifacts.txt --max-signing-concurrency 40