feat: implement GitHub Actions job skipping system (Phase 4)

Implemented source-file-based job skipping with early exit pattern for
GitHub Actions, since it doesn't support CircleCI's dynamic continuation.

Key features:
- SHA-256 hash calculation of source files
- Early exit with 'exit 0' when job already completed for hash
- Skip cache stored in /tmp/cigen_skip_cache
- Architecture-aware skip markers
- Support for @group references and inline patterns
- Completion recording for successful runs

Implementation:
- Created src/providers/github_actions/job_skip.rs with JobSkipGenerator
- Integrated skip steps into workflow generator
- Three-step pattern: calculate hash → check skip → record completion
- Full test coverage with 3 test cases

How it works:
1. Calculate hash of source files and store in $GITHUB_ENV
2. Check if skip marker exists, exit 0 if found (saves ~5-30 sec)
3. Run job normally if not skipped
4. Record completion marker on success

Difference from CircleCI:
- CircleCI: Can completely skip job before container boots (saves minutes)
- GitHub Actions: Job starts but exits early (saves seconds)

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

Author: ndbroadbent

src/providers/github_actions/generator.rs

@@ -1,4 +1,5 @@
 use super::cache::CacheGenerator;
+use super::job_skip::JobSkipGenerator;
 use super::schema::{Job as GHJob, RunsOn, Step, Workflow};
 use crate::models::{Command, Config, Job};
 use miette::{IntoDiagnostic, Result};
@@ -8,12 +9,14 @@ use std::path::Path;
 
 pub struct GitHubActionsGenerator {
     cache_generator: CacheGenerator,
+    job_skip_generator: JobSkipGenerator,
 }
 
 impl GitHubActionsGenerator {
     pub fn new() -> Self {
         Self {
             cache_generator: CacheGenerator::new(),
+            job_skip_generator: JobSkipGenerator::new(),
         }
     }
 
@@ -248,7 +251,6 @@ impl GitHubActionsGenerator {
             });
         }
 
-        // Add automatic cache restoration
         // Determine architecture - use first from architectures or default to amd64
         let architecture = job
             .architectures
@@ -257,16 +259,47 @@ impl GitHubActionsGenerator {
             .map(|s| s.as_str())
             .unwrap_or("amd64");
 
-        let cache_steps = self
-            .cache_generator
-            .generate_cache_steps(config, job, architecture)?;
-        steps.extend(cache_steps);
+        // Add job skip logic if source_files are defined
+        let skip_steps = self
+            .job_skip_generator
+            .generate_skip_steps(config, job, architecture)?;
+
+        if let Some((hash_step, skip_check_step, completion_step)) = skip_steps {
+            // Add hash calculation step
+            steps.push(hash_step);
+
+            // Add skip check step
+            steps.push(skip_check_step);
+
+            // Add automatic cache restoration
+            let cache_steps =
+                self.cache_generator
+                    .generate_cache_steps(config, job, architecture)?;
+            steps.extend(cache_steps);
+
+            // Convert cigen steps to GitHub Actions steps
+            if let Some(job_steps) = &job.steps {
+                for cigen_step in job_steps {
+                    let step = self.build_step(cigen_step)?;
+                    steps.push(step);
+                }
+            }
 
-        // Convert cigen steps to GitHub Actions steps
-        if let Some(job_steps) = &job.steps {
-            for cigen_step in job_steps {
-                let step = self.build_step(cigen_step)?;
-                steps.push(step);
+            // Add completion recording step at the end
+            steps.push(completion_step);
+        } else {
+            // No skip logic - add cache and user steps normally
+            let cache_steps =
+                self.cache_generator
+                    .generate_cache_steps(config, job, architecture)?;
+            steps.extend(cache_steps);
+
+            // Convert cigen steps to GitHub Actions steps
+            if let Some(job_steps) = &job.steps {
+                for cigen_step in job_steps {
+                    let step = self.build_step(cigen_step)?;
+                    steps.push(step);
+                }
             }
         }