fix(agent/agentscripts): create missing log_path parent directory - #28166

Change

• agent/agentscripts/agentscripts.go: in (*Runner).run, after the full logPath resolution and before OpenFile, create the parent directory:

  logDir := filepath.Dir(logPath)
  if err = r.Filesystem.MkdirAll(logDir, 0o700); err != nil {
      return xerrors.Errorf("create script log file directory %q: %w", logDir, err)
  }

Regression test

• agent/agentscripts/agentscripts_test.go: TestExecuteCreatesMissingLogDir runs a script with a nested, nonexistent LogPath and asserts the streamed output and that the log file is created.
• The test uses afero.NewOsFs() on purpose: afero.NewMemMapFs() auto-creates parent directories on OpenFile, so it cannot reproduce the reported failure.
• Verified red without the fix (open .../does/not/exist/install.log: no such file or directory) and green with it.

Local validation

• gofmt clean, go vet, go build, golangci-lint run on the package, and go test -race ./agent/agentscripts/ all pass.

End-to-end

• Validated on a dev instance with a template whose coder_script.log_path targets a nested directory that does not exist. The agent created the parents with mode 0700 and wrote the log file; the workspace agent reported healthy.

Prior attempts

• #22796 and #25545 proposed the same directory-creation approach. Both were closed for non-technical reasons (a low-effort AI PR and a stale community PR), not rejected on the merits. This supersedes them, authored by the issue owner, using 0o700 and adding a regression test.