refactor: Phase 1 - Extract worker interfaces

Created interfaces package to break tight coupling:

1. internal/worker/interfaces/executor.go (30 lines)
   - JobExecutor interface for job execution
   - ExecutionEnv struct for execution context
   - ExecutionResult struct for results

2. internal/worker/interfaces/tracker.go (20 lines)
   - ProgressTracker interface for execution stages
   - StageStart, StageComplete, StageFailed methods
   - JobComplete for final status

3. internal/worker/interfaces/manifest.go (18 lines)
   - ManifestWriter interface for manifest operations
   - Upsert method for update/create
   - BuildInitial method for creating new manifests

These interfaces will enable:
- Dependency injection in future phases
- Mocking for unit tests
- Clean separation between orchestration and execution

Build status: Compiles successfully

internal/worker/interfaces/executor.go

@@ -0,0 +1,33 @@
+// Package interfaces defines the contracts for worker components
+package interfaces
+
+import (
+	"context"
+	"time"
+
+	"github.com/jfraeys/fetch_ml/internal/queue"
+)
+
+// ExecutionEnv holds the environment for job execution
+type ExecutionEnv struct {
+	JobDir        string
+	OutputDir     string
+	LogFile       string
+	GPUDevices    []string
+	GPUEnvVar     string
+	GPUDevicesStr string
+}
+
+// JobExecutor defines the contract for executing jobs
+type JobExecutor interface {
+	// Execute runs a job with the given context, task, and environment
+	Execute(ctx context.Context, task *queue.Task, env ExecutionEnv) error
+}
+
+// ExecutionResult holds the result of job execution
+type ExecutionResult struct {
+	Success  bool
+	ExitCode int
+	Duration time.Duration
+	Error    error
+}

internal/worker/interfaces/manifest.go

@@ -0,0 +1,17 @@
+// Package interfaces defines the contracts for worker components
+package interfaces
+
+import (
+	"github.com/jfraeys/fetch_ml/internal/manifest"
+	"github.com/jfraeys/fetch_ml/internal/queue"
+)
+
+// ManifestWriter defines the contract for writing run manifests
+type ManifestWriter interface {
+	// Upsert updates or creates a manifest in the given directory
+	// The mutate function is called with the current or new manifest
+	Upsert(dir string, task *queue.Task, mutate func(*manifest.RunManifest))
+
+	// BuildInitial creates a new initial manifest for a task
+	BuildInitial(task *queue.Task, podmanImage string) *manifest.RunManifest
+}

internal/worker/interfaces/tracker.go

@@ -0,0 +1,21 @@
+// Package interfaces defines the contracts for worker components
+package interfaces
+
+import (
+	"time"
+)
+
+// ProgressTracker defines the contract for tracking job execution progress
+type ProgressTracker interface {
+	// StageStart marks the beginning of an execution stage
+	StageStart(taskID, stage string)
+
+	// StageComplete marks successful completion of a stage
+	StageComplete(taskID, stage string, duration time.Duration)
+
+	// StageFailed marks a stage failure
+	StageFailed(taskID, stage string, err error)
+
+	// JobComplete marks job completion with final status
+	JobComplete(taskID string, success bool, duration time.Duration)
+}