feat: add MR ID generation for merge queue

Implement GenerateMRID(prefix, branch) to generate merge request IDs
following the convention: <prefix>-mr-<hash>

- Hash derived from branch name + timestamp + random bytes for uniqueness
- Example output: gt-mr-abc123
- Includes deterministic variant for testing (GenerateMRIDWithTime)

Closes: gt-h5n.2

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

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

internal/mq/id.go

@@ -0,0 +1,49 @@
+// Package mq provides merge queue functionality.
+package mq
+
+import (
+	"crypto/rand"
+	"crypto/sha256"
+	"encoding/hex"
+	"fmt"
+	"time"
+)
+
+// GenerateMRID generates a merge request ID following the convention: <prefix>-mr-<hash>
+//
+// The hash is derived from the branch name + current timestamp + random bytes to ensure uniqueness.
+// Example: gt-mr-abc123 for a gastown merge request.
+//
+// Parameters:
+//   - prefix: The project prefix (e.g., "gt" for gastown)
+//   - branch: The source branch name (e.g., "polecat/Nux/gt-xyz")
+//
+// Returns a string in the format "<prefix>-mr-<6-char-hash>"
+func GenerateMRID(prefix, branch string) string {
+	// Generate 8 random bytes for additional uniqueness
+	randomBytes := make([]byte, 8)
+	rand.Read(randomBytes)
+
+	return generateMRIDInternal(prefix, branch, time.Now(), randomBytes)
+}
+
+// GenerateMRIDWithTime generates a merge request ID using a specific timestamp.
+// This is primarily useful for testing to ensure deterministic output.
+// Note: Without randomness, two calls with identical inputs will produce the same ID.
+func GenerateMRIDWithTime(prefix, branch string, timestamp time.Time) string {
+	return generateMRIDInternal(prefix, branch, timestamp, nil)
+}
+
+// generateMRIDInternal is the internal implementation that combines all inputs.
+func generateMRIDInternal(prefix, branch string, timestamp time.Time, randomBytes []byte) string {
+	// Combine branch, timestamp, and optional random bytes for uniqueness
+	input := fmt.Sprintf("%s:%d:%x", branch, timestamp.UnixNano(), randomBytes)
+
+	// Generate SHA256 hash
+	hash := sha256.Sum256([]byte(input))
+
+	// Take first 6 characters of hex-encoded hash
+	hashStr := hex.EncodeToString(hash[:])[:6]
+
+	return fmt.Sprintf("%s-mr-%s", prefix, hashStr)
+}