fix(host): bound host-environment cleanup and reclaim leaked scratch dirs (#1024)

Fixes #1023.

## Problem
In Windows host mode, a single stalled delete syscall (AV/EDR filter driver, unresponsive mount, dying disk) wedged the job forever at `Cleaning up container`. `HostEnvironment.Remove()` bounds every teardown phase (`terminateRunningProcesses`, both `removePathWithRetry` calls) except the `CleanUp` callback — an unbounded `os.RemoveAll(miscpath)` assigned in `startHostEnvironment`. The runner then held its capacity slot indefinitely, the task was reaped as a zombie, and there were no diagnostics.

## Fix
- **Bound the cleanup (availability):** `Remove()` now runs `CleanUp` under `hostCleanupTimeout` (30s) via `runWithTimeout`; on timeout it logs a warning and continues job completion. The stuck goroutine is left to finish (a delete syscall can't be interrupted). Added debug logs around the phase.
- **Reclaim the leak (disk hygiene):** a timed-out cleanup can leave a scratch dir behind, so the existing idle stale-dir sweep is extended to also remove orphaned host-mode scratch dirs (16-hex names) under `Host.WorkdirParent`, leaving the shared `tool_cache` and operator data untouched. The `bind_workdir` gate is dropped from `shouldRunIdleCleanup` so host-mode runners run the sweep.

Reviewed-on: https://gitea.com/gitea/runner/pulls/1024
Reviewed-by: Lunny Xiao <xiaolunwen@gmail.com>

internal/pkg/config/config.example.yaml

@@ -40,11 +40,12 @@ runner:
   # The runner uses exponential backoff when idle, increasing the interval up to this maximum.
   # Set to 0 or same as fetch_interval to disable backoff.
   fetch_interval_max: 5s
-  # While idle, remove stale bind-workdir task directories older than this duration.
-  # Setting either workdir_cleanup_age or idle_cleanup_interval to 0 (or any
-  # non-positive value) disables workdir cleanup entirely.
+  # While idle, remove stale bind-workdir task directories and orphaned host-mode
+  # scratch directories (left behind when a host cleanup delete stalls) older than
+  # this duration. Setting either workdir_cleanup_age or idle_cleanup_interval to 0
+  # (or any non-positive value) disables stale-directory cleanup entirely.
   workdir_cleanup_age: 24h
-  # Cadence for the idle stale bind-workdir cleanup pass.
+  # Cadence for the idle stale-directory cleanup pass.
   idle_cleanup_interval: 10m
   # The base interval for periodic log flush to the Gitea instance.
   # Logs may be sent earlier if the buffer reaches log_report_batch_size