docs: add multi-gateway guide

docs/gateway/multiple-gateways.md

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+---
+summary: "Run multiple Clawdbot Gateways on one host (isolation, ports, and profiles)"
+read_when:
+  - Running more than one Gateway on the same machine
+  - You need isolated config/state/ports per Gateway
+---
+# Multiple Gateways (same host)
+
+Most setups should use one Gateway because a single Gateway can handle multiple messaging connections and agents. If you need stronger isolation or redundancy, run separate Gateways. Both are supported.
+
+## Isolation checklist (required)
+- `CLAWDBOT_CONFIG_PATH` — per-instance config file
+- `CLAWDBOT_STATE_DIR` — per-instance sessions, creds, caches
+- `agents.defaults.workspace` — per-instance workspace root
+- `gateway.port` (or `--port`) — unique per instance
+- Derived ports (bridge/browser/canvas) must not overlap
+
+If these are shared, you will hit config races and port conflicts.
+
+## Recommended: profiles (`--profile`)
+
+Profiles auto-scope `CLAWDBOT_STATE_DIR` + `CLAWDBOT_CONFIG_PATH` and suffix service names.
+
+```bash
+# main
+clawdbot --profile main setup
+clawdbot --profile main gateway --port 18789
+
+# rescue
+clawdbot --profile rescue setup
+clawdbot --profile rescue gateway --port 19001
+```
+
+Per-profile daemons:
+```bash
+clawdbot --profile main daemon install
+clawdbot --profile rescue daemon install
+```
+
+## Port mapping (derived)
+
+Base port = `gateway.port` (or `CLAWDBOT_GATEWAY_PORT` / `--port`).
+
+- `bridge.port = base + 1`
+- `browser.controlUrl port = base + 2`
+- `canvasHost.port = base + 4`
+- Browser profile CDP ports auto-allocate from `browser.controlPort + 9 .. + 108`
+
+If you override any of these in config or env, you must keep them unique per instance.
+
+## Browser/CDP notes (common footgun)
+
+- Do **not** pin `browser.controlUrl` or `browser.cdpUrl` to the same values on multiple instances.
+- Each instance needs its own browser control port and CDP range.
+- If you need explicit CDP ports, set `browser.profiles.<name>.cdpPort` per instance.
+- Remote Chrome: use `browser.profiles.<name>.cdpUrl` (per profile, per instance).
+
+## Manual env example
+
+```bash
+CLAWDBOT_CONFIG_PATH=~/.clawdbot/main.json \
+CLAWDBOT_STATE_DIR=~/.clawdbot-main \
+clawdbot gateway --port 18789
+
+CLAWDBOT_CONFIG_PATH=~/.clawdbot/rescue.json \
+CLAWDBOT_STATE_DIR=~/.clawdbot-rescue \
+clawdbot gateway --port 19001
+```
+
+## Quick checks
+
+```bash
+clawdbot --profile main status
+clawdbot --profile rescue status
+clawdbot --profile rescue browser status
+```