fix(agent): preload manual assessment rules

[M09Ic]

Summary

• preload the foundational aiscan skill for normal agent runtimes
• preserve and deduplicate explicitly loaded skills
• define manual web coverage when users prohibit automated scanners
• add a runtime prompt regression test

Verification

• go test -count=1 ./core/runner ./skills ./cmd/aiscan
• Ubuntu CI-equivalent go test -tags "re2_cgo re2_static" -race -count=1 -timeout 5m ./... (all packages passed except existing network-dependent tools/arsenal test timed out downloading dnsx)
• real deepseek-v4-pro run against a local login fixture: no scanner pseudo-command calls; inspected HTML/JS, form action, client route, and API clues