feat: reactive engine expansion — DAG mutation, ephemeral proxy, HITL gates

[Emperiusm]

Summary

Transforms OpenTools from a static scan pipeline into a reactive, operator-gated execution engine with ephemeral infrastructure support.

Phase A — Dynamic DAG Mutation

• OutputAnalyzer protocol with Nmap + Nuclei analyzers extracting structured intel (services, vulns, URLs) from tool output
• KillChainState accumulates attack surface knowledge across all completed tasks
• MutationStrategy protocol with RedisProbeStrategy auto-pivoting on discovered services
• Engine integration: mutation layer hooks into _mark_completed synchronously — zero race conditions

Phase B — Ephemeral Proxy Routing

• CloudNodeProvider ABC with DigitalOcean + Vultr implementations (httpx, async)
• Vultr provider enforces sshkey_id in creation payload for immediate SSH tunnel auth
• ephemeral_proxy AsyncContextManager with shielded teardown (guaranteed node destruction even under cancellation)
• ProxiedShellExecutor routes NETWORK_ISOLATED tasks through auto-provisioned SOCKS5 proxies
• Orphan sweeper cleans up leaked nodes on startup

Phase C — HITL Approval Gate

• Execution wrapper model (not a separate TaskType) — requires_approval metadata block on any task
• Persistence-first: SQLite is always the source of truth, asyncio.Event is just a notification tripwire
• Write-before-signal: FastAPI routes write decision to DB before calling event.set()
• Read-after-wake: gate executor reads authoritative status from DB, never trusts in-memory state
• Database-owned expiry: approval_expires_at UTC timestamp survives server restarts
• Dedicated approval_gate resource group (9999 slots) — sleeping gates cost zero CPU/RAM
• FastAPI endpoints: list/approve/reject with user-scoped authorization

Review fixes applied:

• Command injection guard on strategy-spawned task hosts (_validate_host regex)
• ApprovalRegistry wired into _active_scans + resource pool configured
• Gate endpoints scoped by user_id via get_scan ownership check

Test plan

• 693 CLI scanner tests passing (34 new mutation + 37 new infra + 16 new approval + 6 engine integration)
• 3 web backend gate route tests passing
• Zero regressions against existing 676 scanner tests
• Existing engine tests (31) pass with mutation + gate additions
• Spec compliance reviewed for Tasks 1, 2, 3, 4, 12 (critical integration points)
• Code quality review on full branch — C1/C2/I7/I8 fixed before merge

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