Webhook receiver: Plane deliveries trigger a tick (PAD-7)

agentd webhook-serve (agentd/webhook.py, stdlib): verifies X-Plane-Signature
(HMAC-SHA256 hex of the body, constant-time; 401 on missing/bad), routes by
data.project to a configured tenant (others 202), coalesces deliveries within
3 s into one tick, runs one more tick when deliveries arrive during a tick,
and retries every 15 s for up to 2 min while the lock is held (tick command
exit 75: flock -E 75 in the cron scripts, cli tick on Busy). Ticks run in a
systemd scope so restarting the receiver never kills one. Log:
<log_dir>/webhook.log; GET /healthz.

Config: webhook section (key names only, PLANE_WEBHOOK_SECRET scrubbed from
child envs like the Plane tokens), notify: {kind: none} as a no-op hook
called on every state move; the unused per-project webhook_secret_key is
gone (Plane webhooks are workspace-level). systemd unit for the LXC and
tools/plane-webhook.py to create/rotate the Plane webhook as the workspace
admin (secret straight into the Vault, never printed).

cli/agentd

@@ -1,5 +1,5 @@
 #!/usr/bin/env python3
-"""Entry point: agentd tick|new|show|comment|state|assign|smoke|check-config (see agentd/cli.py)."""
+"""Entry point: agentd tick|new|show|comment|state|assign|smoke|webhook-serve|check-config (see agentd/cli.py)."""
 import sys
 from pathlib import Path