Add nix flake

[r0hitsharma]

This fixes #21.

Once the PR is merged:

# Direct installation via nix profile
nix profile install github:eljulians/skillfile

# Via home-manager
home.packages = [ inputs.skillfile.packages.${system}.default ];

# Or run directly
nix run github:eljulians/skillfile -- --help

For now you can test with: nix run github:r0hitsharma/skillfile/r0hitsharma/nix-flake -- --version.

For making sure the version and sha's are updated in a consistent manner, the CI process has been updated to have a separate bump, and release step:

flowchart TD                                                                                                                                                                    
      Start[("Last functional commit<br/>on master")] --> Trigger{{"User triggers<br/>bump workflow<br/>(version: 1.4.2)"}}                                                       
                                                                                                                                                                                  
      Trigger --> Bump["<b>Bump Job</b><br/>• Update Cargo.toml<br/>• Update Cargo.lock<br/>• Commit: 'Bump version to v1.4.2'<br/>• Push to master"]                             
                                                                                                                                                                                  
      Bump --> Commit1[("Commit 1:<br/>Version bump")]                                                                                                                            
                                                            
      Commit1 --> Build["<b>Build Job</b><br/>(parallel matrix)<br/>• Build x86_64-linux<br/>• Build aarch64-linux<br/>• Build x86_64-macos<br/>• Build aarch64-macos<br/>• Build 
  x86_64-windows<br/>• Upload artifacts"]                   
                                                                                                                                                                                  
      Build --> CreateRC["<b>Create-RC Job</b><br/>• Download artifacts<br/>• Compute SHA256 hashes<br/>• Update flake.nix<br/>• Commit: 'Update flake.nix to v1.4.2'<br/>• Push  
  to master<br/>• Create v1.4.2-rc tag<br/>• Create pre-release with binaries"]
                                                                                                                                                                                  
      CreateRC --> Commit2[("Commit 2:<br/>flake.nix with hashes<br/><br/><b>Tagged: v1.4.2-rc</b>")]                                                                             
  
      Commit2 -.->|"RC tag triggers<br/>release workflow"| ReleaseStart["<b>Release Workflow Triggered</b>"]                                                                      
                                                            
      ReleaseStart --> CI["<b>CI Job</b><br/>• Run tests<br/>• Run lints"]                                                                                                        
                                                            
      CI -->|"Pass"| Verify["<b>Verify-Install Job</b><br/>• Test install.sh<br/>• Verify binary works"]                                                                          
                                                            
      CI -->|"Fail"| Failed[("❌ Release Aborted<br/>RC remains as pre-release")]                                                                                                 
                                                            
      Verify --> Publish["<b>Publish Job</b><br/>• Publish skillfile-core<br/>• Publish skillfile-sources<br/>• Publish skillfile-deploy<br/>• Publish skillfile"]                
                                                            
      Publish --> Release["<b>Release Job</b><br/>• Download binaries from RC<br/>• Create v1.4.2 tag<br/>• Create full release<br/>• Delete v1.4.2-rc tag<br/>• Delete RC        
  pre-release"]                                             
                                                                                                                                                                                  
      Release --> FinalTag[("Commit 2:<br/>flake.nix with hashes<br/><br/><b>Tagged: v1.4.2</b><br/>(RC tag removed)")]                                                           
  
      FinalTag --> Complete[("✅ Release Complete<br/>• GitHub Release published<br/>• crates.io published<br/>• Nix flake ready")]                                               
                                                            
      style Start fill:#e1f5ff                                                                                                                                                    
      style Commit1 fill:#fff4e1                            
      style Commit2 fill:#e8f5e1                                                                                                                                                  
      style FinalTag fill:#e1ffe8                                                                                                                                                 
      style Complete fill:#d4edda                                                                                                                                                 
      style Failed fill:#f8d7da                                                                                                                                                   
      style Trigger fill:#f0e6ff                                                                                                                                                  
      style ReleaseStart fill:#f0e6ff

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Key Points:

• 2 commits total: One for Cargo.toml/Cargo.lock, one for flake.nix with correct hashes
• RC tag is created on the second commit (which has the correct hashes)
• CI runs on the RC tag before any publishing happens
• If CI fails: Process stops, RC pre-release remains for debugging
• If CI passes: Publishes to crates.io, then promotes RC to full release
• Final state: v1.4.2 tag points to commit with correct hashes, RC is cleaned up

[eljulians]

@r0hitsharma thanks for the new PR! I'll be out starting tomorrow for a week and a half; I want to test this thoroughly because I'm interested in the mechanisms, so I'll wait until then