docs(env): document AI_CHAT_RESUMABLE_STREAM staged-rollout flag (#381)

The resumable-SSE run-stream registry (PR #386/#387) ships behind the
server-side AI_CHAT_RESUMABLE_STREAM env flag, OFF by default: with the
flag off attach always answers 204 and reopened tabs of an active run
fall back to degraded history polling. Document the flag, its default,
its relation to the per-workspace autonomousRuns setting, and the
single-instance constraint in .env.example next to the autonomous-runs
section.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

.env.example

@@ -222,6 +222,18 @@ MCP_DOCMOST_PASSWORD=
 # CLOUD=true) — run a single instance instead. The server logs a startup WARNING
 # when it detects a multi-instance deployment (CLOUD=true) so the constraint is
 # visible, and a startup sweep settles any run left dangling by a restart.
+#
+# Resumable run streams (#184 phase 1.5, #381). With the flag ON, an active
+# durable run tees its SSE frames into an in-memory registry, and a
+# reloaded/second tab attaches via GET /ai-chat/runs/:chatId/stream to follow the
+# run LIVE (replay of the buffered frames + the live tail). With the flag OFF
+# (default) the registry is never populated and attach always answers 204, so a
+# reopened tab of an active run silently falls back to degraded 2.5s history
+# polling — every wire path stays byte-for-byte identical to a build without the
+# feature. Staged-rollout switch: only meaningful when autonomousRuns (above) is
+# enabled for a workspace, and the same single-instance constraint applies (the
+# registry is process-local).
+# AI_CHAT_RESUMABLE_STREAM=false
 
 # --- Anonymous public-share AI assistant ---
 # Opt-in per workspace (AI settings -> "public share assistant"; off by default).