fix(install): promote croniter to a core dependency

Cron is a built-in Hermes feature (CLI `hermes cron`, `cronjob` agent
tool, gateway ticker, scheduler in cron/scheduler.py) but croniter has
been gated behind the [cron] optional extra. Users who do a plain
`pip install hermes-agent` can create jobs via /cron but any recurring
cron schedule silently returns next_run_at=None (HAS_CRONITER=False),
which then gets wrapped into a 'state=error' message only after a tick.

Move croniter into core dependencies so scheduled jobs work out of the
box on any install path. The [cron] extra is kept as an empty
passthrough so existing `pip install hermes-agent[cron]` installs and
the [all]/[termux] extras continue to resolve.

Also update the now-stale user-facing error message in
`compute_next_run()` that still tells users to install `hermes-agent[cron]`.

Salvaged from #17234 (authored by @txbxxx) with a corrected premise:
the original PR claimed [cron] wasn't in [all], but it is (pyproject.toml
line 112). The real UX problem is the plain no-extras install path,
which this fix addresses.

cron/jobs.py

@@ -313,9 +313,10 @@ def compute_next_run(schedule: Dict[str, Any], last_run_at: Optional[str] = None
     elif schedule["kind"] == "cron":
         if not HAS_CRONITER:
             logger.warning(
-                "Cannot compute next run for cron schedule %r: 'croniter' "
-                "is not installed. Install the 'cron' extra (pip install "
-                "'hermes-agent[cron]') to re-enable recurring cron jobs.",
+                "Cannot compute next run for cron schedule %r: 'croniter' is "
+                "not installed. croniter is a core dependency as of v0.9.x; "
+                "reinstall hermes-agent or run 'pip install croniter' in your "
+                "runtime env.",
                 schedule.get("expr"),
             )
             return None