feat: add startup janitor for stale bind-workdir task workspaces (#870)

- Add idle-time cleanup for stale bind-workdir task directories instead of cleaning them on the task execution path.
- Make cleanup behavior configurable with `runner.startup_cleanup_age` as the stale-age threshold (default: `24h`) and `runner.idle_cleanup_interval` as the idle cleanup cadence (default: `10m`).
- Restrict cleanup scope to numeric task directory names only, to avoid touching operator-managed folders.
- Document the cleanup settings in `config.example.yaml` and `README.md`.
- Add tests for stale-directory cleanup, idle cleanup throttling, and config default/override parsing.

## Why

When a runner or host crashes, normal per-task cleanup may not run, leaving stale task directories under the bind-workdir root. Running this cleanup only while the runner is idle recovers that disk space without adding overhead to active job execution.

If you want, I can also tighten the wording around `startup_cleanup_age`, since the key name now reads a bit misleadingly relative to the actual behavior.

---------

Co-authored-by: silverwind <me@silverwind.io>
Reviewed-on: https://gitea.com/gitea/runner/pulls/870
Reviewed-by: silverwind <2021+silverwind@noreply.gitea.com>

internal/app/poll/poller.go

@@ -27,6 +27,11 @@ type TaskRunner interface {
 	Run(ctx context.Context, task *runnerv1.Task) error
 }
 
+// IdleRunner can run maintenance while the poller is idle.
+type IdleRunner interface {
+	OnIdle(ctx context.Context)
+}
+
 type Poller struct {
 	client       client.Client
 	runner       TaskRunner
@@ -95,6 +100,7 @@ func (p *Poller) Poll() {
 
 		task, ok := p.fetchTask(p.pollingCtx, s)
 		if !ok {
+			p.runIdleMaintenance()
 			<-sem
 			if !p.waitBackoff(s) {
 				return
@@ -119,6 +125,7 @@ func (p *Poller) PollOnce() {
 	for {
 		task, ok := p.fetchTask(p.pollingCtx, s)
 		if !ok {
+			p.runIdleMaintenance()
 			if !p.waitBackoff(s) {
 				return
 			}
@@ -130,6 +137,12 @@ func (p *Poller) PollOnce() {
 	}
 }
 
+func (p *Poller) runIdleMaintenance() {
+	if idleRunner, ok := p.runner.(IdleRunner); ok {
+		idleRunner.OnIdle(p.jobsCtx)
+	}
+}
+
 func (p *Poller) Shutdown(ctx context.Context) error {
 	p.shutdownPolling()