Compute OIDC auth, async Horde agents, and orchestrator improvements (#913)

Rework of the Horde agent subsystem from synchronous per-thread I/O to an async ASIO-driven architecture, plus provisioner scale-down with graceful draining, OIDC authentication, scheduler improvements, and dashboard UI for provisioner control.

### Async Horde Agent Rewrite
- Replace synchronous `HordeAgent` (one thread per agent, blocking I/O) with `AsyncHordeAgent` — an ASIO state machine running on a shared `io_context` thread pool
- Replace `TcpComputeTransport`/`AesComputeTransport` with `AsyncTcpComputeTransport`/`AsyncAesComputeTransport`
- Replace `AgentMessageChannel` with `AsyncAgentMessageChannel` using frame queuing and ASIO timers
- Delete `ComputeBuffer` and `ComputeChannel` ring-buffer classes (no longer needed)

### Provisioner Drain / Scale-Down
- `HordeProvisioner` can now drain agents when target core count is lowered: queries each agent's `/compute/session/status` for workload, selects candidates by largest-fit/lowest-workload, and sends `/compute/session/drain`
- Configurable `--horde-drain-grace-period` (default 300s) before force-kill
- Implement `IProvisionerStateProvider` interface to expose provisioner state to the orchestrator HTTP layer
- Forward `--coordinator-session`, `--provision-clean`, and `--provision-tracehost` through both Horde and Nomad provisioners to spawned workers

### OIDC Authentication
- `HordeClient` accepts an `AccessTokenProvider` (refreshable token function) as alternative to static `--horde-token`
- Wire up `OidcToken.exe` auto-discovery via `httpclientauth::CreateFromOidcTokenExecutable` with `--HordeUrl` mode
- New `--horde-oidctoken-exe-path` CLI option for explicit path override

### Orchestrator & Scheduler
- Orchestrator generates a session ID at startup; workers include `coordinator_session` in announcements so the orchestrator can reject stale-session workers
- New `Rejected` action state — when a remote runner declines at capacity, the action is rescheduled without retry count increment
- Reduce scheduler lock contention: snapshot pending actions under shared lock, sort/trim outside the lock
- Parallelize remote action submission across runners via `WorkerThreadPool` with slow-submit warnings
- New action field `FailureReason` populated by all runner types (exit codes, sandbox failures, exceptions)
- New endpoints: `session/drain`, `session/status`, `session/sunset`, `provisioner/status`, `provisioner/target`

### Remote Execution
- Eager-attach mode for `RemoteHttpRunner` — bundles all attachments upfront in a `CbPackage` for single-roundtrip submits
- Track in-flight submissions to prevent over-queuing
- Show remote runner hostname in `GetDisplayName()`
- `--announce-url` to override the endpoint announced to the coordinator (e.g. relay-visible address)

### Frontend Dashboard
- Delete standalone `compute.html` (925 lines) and `orchestrator.html` (669 lines), consolidated into JS page modules
- Add provisioner panel to orchestrator dashboard: target/active/estimated core counts, draining agent count
- Editable target-cores input with debounced POST to `/orch/provisioner/target`
- Per-agent provisioning status badges (active / draining / deallocated) in the agents table
- Active vs total CPU counts in agents summary row

### CLI
- New `zen compute record-start` / `record-stop` subcommands
- `zen exec` progress bar with submit and completion phases, atomic work counters, `--progress` mode (Pretty/Plain/Quiet)

### Other
- `DataDir` supports environment variable expansion
- Worker manifest validation checks for `worker.zcb` marker to detect incomplete cached directories
- Linux/Mac runners `nice(5)` child processes to avoid starving the main server
- `ComputeService::SetShutdownCallback` wired to `RequestExit` via `session/sunset`
- Curl HTTP client logs effective URL on failure
- `MachineInfo` carries `Pool` and `Mode` from Horde response
- Horde bundle creation includes `.pdb` on Windows

1 files changed, 37 insertions, 1 deletions

diff --git a/src/zennomad/nomadclient.cpp b/src/zennomad/nomadclient.cpp
index 9edcde125..4bb09a930 100644
--- a/src/zennomad/nomadclient.cpp
+++ b/src/zennomad/nomadclient.cpp
@@ -58,7 +58,11 @@ NomadClient::Initialize()
 }
 
 std::string
-NomadClient::BuildJobJson(const std::string& JobId, const std::string& OrchestratorEndpoint) const
+NomadClient::BuildJobJson(const std::string& JobId,
+						  const std::string& OrchestratorEndpoint,
+						  const std::string& CoordinatorSession,
+						  bool				 CleanStart,
+						  const std::string& TraceHost) const
 {
 	ZEN_TRACE_CPU("NomadClient::BuildJobJson");
 
@@ -94,6 +98,22 @@ NomadClient::BuildJobJson(const std::string& JobId, const std::string& Orchestra
 			IdArg << "--instance-id=nomad-" << JobId;
 			Args.push_back(std::string(IdArg.ToView()));
 		}
+		if (!CoordinatorSession.empty())
+		{
+			ExtendableStringBuilder<128> SessionArg;
+			SessionArg << "--coordinator-session=" << CoordinatorSession;
+			Args.push_back(std::string(SessionArg.ToView()));
+		}
+		if (CleanStart)
+		{
+			Args.push_back("--clean");
+		}
+		if (!TraceHost.empty())
+		{
+			ExtendableStringBuilder<128> TraceArg;
+			TraceArg << "--tracehost=" << TraceHost;
+			Args.push_back(std::string(TraceArg.ToView()));
+		}
 		TaskConfig["args"] = Args;
 	}
 	else
@@ -115,6 +135,22 @@ NomadClient::BuildJobJson(const std::string& JobId, const std::string& Orchestra
 			IdArg << "--instance-id=nomad-" << JobId;
 			Args.push_back(std::string(IdArg.ToView()));
 		}
+		if (!CoordinatorSession.empty())
+		{
+			ExtendableStringBuilder<128> SessionArg;
+			SessionArg << "--coordinator-session=" << CoordinatorSession;
+			Args.push_back(std::string(SessionArg.ToView()));
+		}
+		if (CleanStart)
+		{
+			Args.push_back("--clean");
+		}
+		if (!TraceHost.empty())
+		{
+			ExtendableStringBuilder<128> TraceArg;
+			TraceArg << "--tracehost=" << TraceHost;
+			Args.push_back(std::string(TraceArg.ToView()));
+		}
 		TaskConfig["args"] = Args;
 	}