readPump/writePump read the shared c.ws field on every op, so on reconnect
connectToURL swapped in a new socket and started a second pair of pumps
without stopping the old ones — the stale writePump then wrote to the new
socket concurrently with the new writePump, panicking with "concurrent write
to websocket connection".

Each connection now owns its handles: pumps capture their conn (and a
per-connection quit/done) as locals, connectToURL signals the previous
connection's pumps to stop and closes the old socket under a new connMu
before swapping in the new one, and Close tears down under the same lock.

Adds a reconnect regression test and makes the wsbridge test mock thread-safe
so the suite is race-clean.

Per-channel config is now: `enabled` (bool, defaults true — set false to keep
a channel configured but turn delivery off) plus a required, explicit `events`
list. Valid event values are mission_completed, mission_failed, mission_stopped,
or "all" (expands to every terminal event). Both gateway and command_center use
the same shape; channel is the only gateway-only extra.

Omitting `events` is now a validation error rather than silently defaulting to
all — delivery is always explicit.

Add missions/notifications.mdx covering the per-mission `notification` block —
gateway + command_center channels, the enabled toggle (default true), the
required explicit `events` list with the "all" shorthand, the gateway-only
channel override, what each terminal event carries, and validation rules.
Register it in the missions nav and cross-link from the gateways and
command_center config pages.

mission_stopped is no longer a notification event: it's removed from the valid
events set (so "all" expands to mission_completed + mission_failed), and a
user-initiated stop no longer dispatches a notification. mission_stopped
remains a normal mission-lifecycle event on the command-center event stream —
this only affects the notification feature. Docs + tests updated.

When a gateway is configured, agents get a builtins.gateway.post tool that
posts a message (with an optional channel name/id override) to the gateway's
external system via the new SDK PostMessage RPC. The gateway Manager satisfies
aitools.GatewayBridge directly; the bridge threads through the runner and agent
options alongside the human-input bridge.

The tool is only a valid reference when a gateway block is present (rejected by
squadron verify otherwise) and is hidden from the command-center tool list when
no gateway is configured. Tests + gateways doc added.

The gateway Manager fetches the gateway's MessageToolSpec on startup; the
GatewayBridge exposes its description + JSON Schema, and builtins.gateway.post
surfaces them to the LLM (raw-schema passthrough) and forwards the agent's raw
payload to the gateway. Lets each gateway define its own rich-message shape
(text, embeds/blocks, attachments). Tests + gateways doc updated.

…DMEs)

The post tool now owns an `attachments` field (injected into the gateway
schema) taking {slot, path} references into the mission's memory/scratchpad.
Squadron resolves each, reads the bytes (25 MiB/file cap), strips them from
the gateway payload, and ships them to the gateway to upload. Eliminates the
SSRF surface of fetching arbitrary model-chosen URLs. Bump SDK to v0.0.3.