feat(admin): plugin write plane — admin.kick/ban/unban/broadcast

Phase 1 (plugin side) of docs/ADMIN_CONTROLS.md: a staff write plane so the
website can moderate the live shard.

- BridgeAdmin.cs: inbound admin.kick, admin.ban (timed + indefinite),
  admin.unban, admin.broadcast. Each requires an `actor`, refuses targets at
  or above AdminAccessFloor (default CoOwner — Owner-only shield), replies
  admin.ok/admin.error with the reqId echoed, and emits an admin.audit
  (origin=web) broadcast. Attribution is web:<actor> in the console log and
  the ban BanDealer tag. Kicking enumerates NetState.Instances so a
  character-select session is caught too.
- BridgeConfig/Bridge.cfg: AdminWriteEnabled (default OFF — opt-in),
  AdminAccessFloor, broadcast/reason length caps, ban duration clamp.
- tools/stub_sidecar_admin.ps1: live smoke-test harness; *.log gitignored.

Verified: compiles clean against ServUO (0 err/warn); live run on the seeded
shard confirms all four verbs, the audit stream, timed-ban fields, and the
Owner-floor refusal, with no exceptions.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_0114TpmrNW4wNXsHq5CR72jQ

overlay/Config/Bridge.cfg

@@ -29,6 +29,24 @@ TownCrierMaxLineLength=200
 TownCrierMaxActive=20
 TownCrierMaxDurationSec=86400
 
+# Admin write plane (staff moderation from the website). OFF by default: the whole
+# feature is opt-in per shard. When enabled, inbound admin.* commands (kick/ban/unban/
+# broadcast) are honored. Authorization is enforced on the website; the shard trusts the
+# loopback socket and applies a hard floor below.
+AdminWriteEnabled=false
+
+# The one shard-side safety floor. An admin.* command refuses any target whose AccessLevel
+# is at or above this, so even a compromised sidecar can never touch the Owner. Values are
+# AccessLevel names (Player, VIP, Counselor, Decorator, Spawner, GameMaster, Seer,
+# Administrator, Developer, CoOwner, Owner). Default CoOwner => only Owner/CoOwners shielded.
+AdminAccessFloor=CoOwner
+
+# Defense-in-depth caps on admin.* payloads (mirroring the town-crier caps).
+AdminBroadcastMaxLength=300
+AdminReasonMaxLength=400
+# Clamp on a timed ban's duration, seconds. A ban with no/zero duration is indefinite.
+AdminBanMaxDurationSec=31536000
+
 # The test scaffolding in tools/scaffolding/ reads its own flags from this file
 # (SeedOnStart, CensusOnStart, ProbeOnStart). They are absent here on purpose:
 # Config.Get returns the default of false when a key is missing, so a deployed