fix(cron): SSH dispatch validation + failure detection (#350)

VPS agent dispatch reported OK while remote hermes binary paths were
broken (/root/wizards/.../venv/bin/hermes: No such file or directory).

Root causes:
1. No validation that remote hermes binary exists before dispatch
2. Scheduler failure detection missed common SSH error patterns

New cron/ssh_dispatch.py:
- DispatchResult: structured result with success/failure, exit_code,
  stderr, human-readable failure_reason
- SSHEnvironment: validates remote hermes binary via SSH probe (test -x)
  before dispatch; caches validated path; proper timeout/error handling
- dispatch_to_hosts(): multi-host dispatch returning per-host results
- format_dispatch_report(): human-readable summary

cron/scheduler.py _SCRIPT_FAILURE_PHRASES expanded:
- no such file or directory (exact bash error)
- command not found
- hermes binary not found / hermes not found
- ssh: connect to host
- connection timed out
- host key verification failed

These are detected by _detect_script_failure() so broken SSH dispatches
are flagged as failures instead of reported as OK.

Closes #350

cron/ssh_dispatch.py

@@ -18,16 +18,14 @@ from typing import Optional
 
 logger = logging.getLogger(__name__)
 
-# Default timeout for SSH commands (seconds)
 _SSH_TIMEOUT = int(os.getenv("HERMES_SSH_TIMEOUT", "30"))
 
-# Default hermes binary paths to probe on remote hosts
 _DEFAULT_HERMES_PATHS = [
     "/root/wizards/{agent}/venv/bin/hermes",
     "/root/.local/bin/hermes",
     "/usr/local/bin/hermes",
     "~/.local/bin/hermes",
-    "hermes",  # fallback to PATH
+    "hermes",
 ]
 
 
@@ -74,7 +72,6 @@ class DispatchResult:
 
     @property
     def failure_reason(self) -> str:
-        """Human-readable failure reason."""
         if self.success:
             return ""
         if self.error:
@@ -91,12 +88,6 @@ class SSHEnvironment:
 
     Validates remote hermes binary paths before dispatching and returns
     structured results so callers can distinguish success from failure.
-
-    Usage:
-        ssh = SSHEnvironment(host="root@ezra", agent="allegro")
-        result = ssh.dispatch("--help")
-        if not result.success:
-            logger.error("Dispatch failed: %s", result.failure_reason)
     """
 
     def __init__(
@@ -117,7 +108,6 @@ class SSHEnvironment:
         self._validated_path: str = ""
 
     def _ssh_base_cmd(self) -> list[str]:
-        """Build the base SSH command."""
         cmd = ["ssh", "-o", "StrictHostKeyChecking=accept-new"]
         cmd.extend(["-o", "ConnectTimeout=10"])
         cmd.extend(["-o", "BatchMode=yes"])
@@ -129,7 +119,6 @@ class SSHEnvironment:
         return cmd
 
     def _resolve_hermes_paths(self) -> list[str]:
-        """Return candidate hermes binary paths for the remote host."""
         if self.hermes_path:
             return [self.hermes_path]
         paths = []
@@ -269,7 +258,7 @@ def format_dispatch_report(results: dict[str, DispatchResult]) -> str:
         status = "OK" if result.success else "FAILED"
         line = f"  {host}: {status}"
         if not result.success:
-            line += f" — {result.failure_reason}"
+            line += f" -- {result.failure_reason}"
         if result.duration_ms:
             line += f" ({result.duration_ms}ms)"
         lines.append(line)