fetch: every outbound request carries one generic browser User-Agent

Stripping forwarding headers kept our IP and chain out of reach, but the
User-Agent went through untouched. Clients that fell back to their own name
("crawlkit-roles", "crawlkit-virk", ...) tied all ten exits together as one
actor, and one missing UA meant Go-http-client. The proxy now overwrites the
UA on every request (WEIRCON_USER_AGENT, default current Firefox) and the
egress prober uses the same string.

Also dropped: From, Sec-Ch-Ua* (would contradict a Firefox UA) and any
X-Weircon-* by prefix. New end-to-end test drives the whole handler and
checks what the target actually receives.

Claude-Session: https://claude.ai/code/session_01AUj1LSMmrowcomkn1ciAgj

README.md

@@ -130,6 +130,16 @@ curl -H "X-WEIRCON-RANDOM-IP: $API_KEY" \
 
 Status, headers, and body of the upstream response are returned directly. The service adds `X-WEIRCON-EGRESS-PROXY: <id>` so the caller can see which tunnel was used.
 
+### What the target sees
+
+Every outbound request is anonymized before it leaves a tunnel, whatever the caller sent:
+
+- **User-Agent is always replaced** with one generic desktop-browser string (`WEIRCON_USER_AGENT`, default a current Firefox). A client that forgets to set a UA, or sets its own name, can't identify you. The background egress prober uses the same UA.
+- **Removed:** our control headers (`X-Weircon-*`), every forwarding/client-IP header (`Forwarded`, `Via`, `X-Forwarded-*`, `X-Real-Ip`, `Cf-*`, `True-Client-Ip`, …), `From`, and `Sec-Ch-Ua*` client hints (they would contradict a Firefox UA).
+- **Passed through untouched:** `Accept*`, `Cookie`, `Referer`, `Authorization` and the body. What you put there is yours to keep clean.
+
+Bump `WEIRCON_USER_AGENT` every few browser releases; a years-old version is a bot tell of its own.
+
 ### Live status (`/status`)
 
 `GET /status` returns a poll-friendly JSON snapshot for scraper-side dashboards — cheap to produce (no network I/O in the request path; egress IPs come from a background prober) and safe to hit every 1–5 s. It's behind the same API-key gate as `/`.