fix: create parent directory for coder_script log_path - #22796

…8166)

Previously, a `coder_script` whose `log_path` pointed under a directory
that did not yet exist failed before the script ran, with no per-script
log output. `OpenFile(logPath, O_CREATE|O_RDWR, 0o600)` creates the log
file but not its parent directories, so the open returned `ENOENT`. The
failure only surfaced in the agent log (`startup script(s) failed` /
`shutdown script(s) failed`) and never reached the script's own UI logs,
which made it look like a silent failure.

This creates the resolved parent directory with
`MkdirAll(filepath.Dir(logPath), 0o700)` before opening the log file, so
the script runs and its log is written. `0o700` matches the existing
script data-dir and secret-file directory conventions in this package.
Resolution of `~`, environment variables, and paths relative to `LogDir`
is unchanged; only the parent directory is now created.

Fixes #21986

<details><summary>Implementation notes and validation</summary>

**Change**

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

  ```go
  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](<#22796>) and
[#25545](<#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.

</details>

---

*Raised on behalf of* @35C4n0r *by Coder Agents.*