fix(mcp): harden the streamable HTTP listener's session replacement and shutdown

Addresses the automated review findings on the PR: refuse requests once the
session slot is closed and join an arming initialize during shutdown; retry
failed close stages instead of replaying the rejection; classify loopback
binds by address class and add the bind's own hostname to the Host and Origin
allowlist; let only a POST with the streamable headers replace an open
session; announce the bound URL at every log level; route `mcp -h --http` to
the HTTP entry.

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>

docs/cli.md

@@ -322,8 +322,8 @@ rea mcp --http --port 8765
 
 `--host` and `--port` default to `MCP_HOST` and `MCP_PORT` from the environment,
 then to `127.0.0.1` and an ephemeral port. The bound URL is written to stderr as
-one JSON log line (`"msg":"MCP transport listening"`); nothing is written to
-stdout. `rea mcp --http --help` lists the accepted options. The listener serves
+one JSON log line (`"msg":"MCP transport listening"`) whatever `REA_LOG_LEVEL`
+is set to; nothing is written to stdout. `rea mcp --http --help` lists the accepted options. The listener serves
 one client at a time: a session ends with the client's `DELETE`, and a client
 that connects while another session is open takes it over, so a client that
 exited without ending its session needs no restart. The listener runs until