Add SSRF-guarded web fetch agent tool

* feat(web-fetch): add web_fetch tool to read a specific URL's content

* test(web-fetch): add SSRF coverage and fail closed on empty DNS resolution

Add explicit SSRF regression tests for the web_fetch path covering
loopback, private LAN ranges, link-local/metadata, IPv6 private/local,
redirect-into-private, and unsupported schemes. Harden _public_http_url
to fail closed when a hostname resolves to no addresses.

src/agent_loop.py

@@ -199,6 +199,12 @@ Or with JSON for fresh news:
 ```
 Search the web for a SINGLE quick fact/lookup mid-task. For news / "today" / "latest" queries, pass `time_filter` ("day", "week", "month", or "year"). NOT for "research X" / "do research on X" / "look into X" requests — those mean a multi-source DEEP RESEARCH job: use `trigger_research` instead (it runs in the Deep Research sidebar and produces a full report). web_search = one quick query; trigger_research = a researched report.""",
 
+    "web_fetch": """\
+```web_fetch
+<url or domain>
+```
+Fetch and read the text content of a SPECIFIC URL the user names (e.g. "check example.com", "what does this page say <url>"). A bare domain like `example.com` works (defaults to https). Use this when you already have a concrete URL. For open-ended lookups use `web_search`, and for "research X" jobs use `trigger_research`.""",
+
     "read_file": """\
 ```read_file
 <file path>