feat: Dagger module for local development

[MChorfa]

Adds minimal Dagger functions for local dev (build, test, lint). Complements existing CI without replacing it.

[MChorfa]

Major Improvements to Dagger Module

This PR has been significantly enhanced from the original basic implementation:

What is new:

Caching & Performance:

• Added Go module cache (/go/pkg/mod) and build cache (/go/build-cache) for faster incremental builds
• Reduced container startup time across all operations

Artifact Outputs (not just stdout):

• All functions now return proper *dagger.Directory / *dagger.File artifacts instead of string output
• Enables downstream pipeline composition and artifact export

Cross-Platform Builds:

• BuildForPlatform(os, arch) - Build for linux/darwin/windows × amd64/arm64
• Supports reproducible cross-compilation with version injection via ldflags

Security & Supply Chain:

• VulnScan() - Runs govulncheck for vulnerability scanning
• SBOM(format) - Generates CycloneDX/SPDX SBOMs using Syft
• Provenance(os, arch) - Generates SHA256 checksums for binary provenance
• These align with SLSA Level 4+ and evidence-native engineering principles

Linting:

• Lint() now uses golangci-lint with structured JSON output instead of raw shell script

Complete Pipeline:

• All() - Runs the full CI suite: tests, build, lint, vuln-scan, and SBOM generation in one call

Technical Updates:

• Updated Dagger SDK from v0.19.11 to v0.20.6
• Aligned Go version with project (1.25)
• Updated engine version in dagger.json

Ready for review! 🚀