Stop misclassifying innocuous shell redirections (1.4.0)

[gmpassos]

Context

Agents routinely append innocuous redirections to commands, and the analyzer mishandled the whole family — 2>&1, 1>&2, 2>/dev/null, >/dev/null, &>/dev/null, >/dev/null 2>&1.

Two concrete defects:

1. 2>&1 was mis-parsed. The bash tokenizer only knew 2>/2>>, so cmd 2>&1 produced a target-less 2> redirection, a spurious "Redirection without a target" diagnostic, and a phantom CommandInvocation with executable 1. 1>&2 and &>/dev/null broke the same way.
2. False writeFilesystem. The phantom errorOutput node (and even well-formed >/dev/null) added a writeFilesystem capability → modifyFiles effect, pushing read-only commands toward REVIEW.

Changes

• AST (command_node.dart): new RedirectionType.mergeStreams, combinedOutput, combinedAppendOutput.
• All three parsers (shell_parser, windows_cmd_parser, powershell_parser): recognize fd-duplication (N>&M, >&-) and combined redirects (&>, &>>). Merges are self-contained — no consumed word, no phantom command, no diagnostic. Leading-digit fds only trigger when immediately followed by > (so 2foo stays a word).
• Capability detector: fd-merges add nothing; null-sink targets (/dev/null, NUL, $null) no longer add writeFilesystem/readFilesystem. Genuine targets (> out.txt, &> out.log) still do.
• BenignRedirectionDetector (new, in the default suite): records merges and null-sink discards as safe-level benign-redirection findings for audit visibility. Decision-neutral since safe < mediumRisk.
• DangerousOperatorDetector: recognizes >& / &> as redirection sequences so fd-dups aren't mislabeled as output redirection.
• Version bumped to 1.4.0 + CHANGELOG entry.

Verification

• dart analyze clean; dart format applied; 428 tests pass (+24 new across parser, capability, security, and end-to-end suites).
• End-to-end sanity check: ls -la 2>&1, grep x f >/dev/null, foo &>/dev/null, cmd >/dev/null 2>&1 → allow, no modifyFiles, benign marker present; rm -rf / >/dev/null 2>&1 → deny (command risk not masked); echo hi > out.txt → still modifyFiles.

Limits (documented in code)

• Null sinks limited to /dev/null, NUL/NUL:, $null.
• Only single-digit source fds are modeled (12>&3 is not treated as a merge).

🤖 Generated with Claude Code

[codecov]

Codecov Report

❌ Patch coverage is 77.18447% with 47 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
lib/src/parser/powershell_parser.dart	67.30%	17 Missing ⚠️
lib/src/parser/windows_cmd_parser.dart	66.00%	17 Missing ⚠️
...ecurity/detectors/dangerous_operator_detector.dart	42.85%	8 Missing ⚠️
lib/src/parser/shell_parser.dart	92.45%	4 Missing ⚠️
...ecurity/detectors/benign_redirection_detector.dart	96.29%	1 Missing ⚠️

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