READMEs use deprecated ngrok `--host-header=rewrite`; switch to traffic policy (or drop it)

[zimeg]

What

Recent ngrok versions print a deprecation warning for the --host-header flag:

Flag --host-header has been deprecated, use traffic policy instead

ngrok is moving Host-header rewriting off the dedicated flag onto its rule-based traffic policy system. The flag will eventually be removed.

Where we use it

Three example READMEs instruct developers to run the deprecated form:

• ai/slackbot-mcp-client/no-auth/README.md — ngrok http 3000 --host-header=rewrite
• ai/slackbot-mcp-client/rich-responses/mcp-apps/README.md — ngrok http 3000 --host-header=rewrite
• ai/slackbot-mcp-client/slack-identity/README.md — ngrok http 3000 --host-header=rewrite

So anyone following these examples' setup steps hits the deprecation warning today, and the command will break when ngrok removes the flag.

(For contrast, SCRATCHPAD.md already uses the bare ngrok http 3000 form — unaffected.)

Suggested fix

Update the three READMEs. Two options:

1. Drop the flag (simplest — verify it still works): ngrok http 3000. Slack/Bolt apps generally don't need Host rewriting; the local receiver doesn't inspect Host. If the examples run fine without it, this is the cleanest fix.
2. If Host rewriting is actually needed, replace it with a traffic policy:

   # policy.yml  — ngrok http 3000 --traffic-policy-file=policy.yml
   on_http_request:
     - actions:
         - type: add-headers
           config:
             headers:
               host: localhost:3000

Recommend confirming option 1 works (run each example end-to-end without the flag) before committing to it.

References

• ngrok traffic policy: https://ngrok.com/docs/traffic-policy/
• Warning text: Flag --host-header has been deprecated, use traffic policy instead