Centralized AST walker for consistent bash analysis

[ldayton]

Summary

Replace scattered partial AST traversals with a single recursive walker that analyzes all bash constructs consistently.

Problem

The old code had multiple scattered functions that partially traversed the AST:

• extract_simple_commands
• has_output_redirect
• get_command_substitutions
• split_pipeline
• split_command_list

This led to inconsistent handling:

• Config rules didn't reach inside compound commands
• Process substitution (>()) was silently allowed without checking inner commands
• Function definitions weren't analyzed

Solution

New src/dippy/core/analyzer.py with a single analyze() function that recursively walks the entire AST:

def analyze(command: str, config: Config, cwd: Path) -> Decision:
    """Single recursive walker. Unknown nodes default to ask."""
    
    match node.kind:
        case "command": return analyze_command(node, config, cwd)
        case "pipeline": return combine([analyze(cmd) for cmd in node.commands])
        case "list": return combine([analyze(p) for p in parts])
        case "if": return combine([analyze(condition), analyze(then_body), ...])
        case "while" | "until": return combine([analyze(condition), analyze(body)])
        case "for" | "select": return analyze(body)
        case "case": return combine([analyze(pattern.body) for pattern in patterns])
        case "function": return analyze(body)
        case "subshell" | "brace-group": return analyze(body)
        case "time": return analyze(pipeline)
        case _: return Decision("ask", f"unrecognized: {node.kind}")

Key Improvements

Before	After
if true; then rm -rf /; fi → ask "if"	→ matches deny rm -rf /*
diff <(ls) >(tee /etc/passwd) → allow	→ ask (inner cmd analyzed)
foo() { rm -rf /; } → ask "foo()"	→ matches deny rules
echo > file.txt → "output redirect"	→ "redirect to file.txt"

Changes

• Add src/dippy/core/analyzer.py (new file, ~350 lines)
• Simplify dippy.py from 617 to 286 lines
• Add read to SIMPLE_SAFE (shell builtin)
• Handle Parable's time node type
• Update tests to reflect improved behavior

Test Plan

• All 9286 tests pass on Python 3.11, 3.12, 3.13, 3.14
• just check passes (lint, fmt, lock)