merge: resolve conflicts with main (workdir reuse)

Main added execenv.Reuse() for workdir reuse across tasks on the same
issue. Our branch removed Type/BranchName/gitRoot from Environment
(repos are now checked out on demand). Resolution: keep Reuse() but
simplify it to work with the new Environment struct (no workspace type
tracking). Keep the "reused" log field from main, drop removed fields.

server/internal/daemon/execenv/execenv.go

@@ -116,6 +116,28 @@ func Prepare(params PrepareParams, logger *slog.Logger) (*Environment, error) {
 	return env, nil
 }
 
+// Reuse wraps an existing workdir into an Environment and refreshes context files.
+// Returns nil if the workdir does not exist (caller should fall back to Prepare).
+func Reuse(workDir, provider string, task TaskContextForEnv, logger *slog.Logger) *Environment {
+	if _, err := os.Stat(workDir); err != nil {
+		return nil
+	}
+
+	env := &Environment{
+		RootDir: filepath.Dir(workDir),
+		WorkDir: workDir,
+		logger:  logger,
+	}
+
+	// Refresh context files (issue_context.md, skills).
+	if err := writeContextFiles(workDir, provider, task); err != nil {
+		logger.Warn("execenv: refresh context files failed", "error", err)
+	}
+
+	logger.Info("execenv: reusing env", "workdir", workDir)
+	return env
+}
+
 // Cleanup tears down the execution environment.
 // If removeAll is true, the entire env root is deleted. Otherwise, workdir is
 // removed but output/ and logs/ are preserved for debugging.