feat: add scanmalware search agent

[jonaslejon]

Proposed changes

Adds ScanMalware as a search agent. No API key: the API is
anonymous, so it works on a fresh install with no config. The query is SMQL, ScanMalware's own
syntax, passed through unchanged the way fofa and shodan pass theirs.

uncover -scanmalware 'domain:*.example.com'
uncover -scanmalware 'url:https://example.com/login'
uncover -scanmalware 'asn:15169 AND country:DE'

Two things for review:

• It returns URLs and hostnames, not ip:port. The archive records what a browser saw, and a
  scan resolves several addresses, so there is no single listening service. Host and Url are
  set, IP/Port left empty, which runner.go already handles by falling back to host;
  nerdydata sets the same subset.
• One change outside the agent. shodan-idb was the only keyless engine and is named as a
  literal in two places. I replaced those with an anonymousAgents set; happy to use two literals
  instead if you'd rather keep the diff minimal.

Proof

go build, go test ./... and golangci-lint run --timeout=30m ./... clean on dev with Go
1.24.6, matching what CI installs from go.mod. Measured from an empty $HOME, no config:

domain:*.stripe.com        32 results   1.61s
asn:15169 AND country:DE   92 results   2.03s
has:malware                94 results   3.82s

Seven tests, each first run against a deliberately broken build to confirm it fails. That is how
the cancellation test got rewritten: the first version drained the channel, so it passed even with
sources.SendResult swapped for a plain results <- r.

Checklist

• Pull request is created against the dev branch
• All checks passed
• I have added tests that prove my feature works
• I have added necessary documentation (README engine list, -e help, -scanmalware/-sm flag)

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Disclosure: I run ScanMalware, so this is a vendor-submitted source. Flagging it up front
rather than leaving it to be found in review.

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