feat: add bash validation submodules — readOnlyValidation, destructiveCommandWarning, modeValidation, sedValidation, pathValidation, commandSemantics

Ports 6 of 9 upstream BashTool validation submodules:
- readOnlyValidation: blocks write/state-modifying commands in read-only mode
- destructiveCommandWarning: flags dangerous commands (rm -rf /, fork bombs, etc.)
- modeValidation: enforces permission mode constraints on commands
- sedValidation: blocks sed -i in read-only mode
- pathValidation: detects directory traversal and home dir escapes
- commandSemantics: classifies command intent (read-only, write, destructive, network, etc.)

Full validation pipeline: validate_command() runs all checks in priority order.
32 new tests covering all validation paths.

Remaining bash submodules for separate lane: bashPermissions, bashSecurity, shouldUseSandbox

rust/crates/runtime/src/lib.rs

@@ -1,4 +1,5 @@
 mod bash;
+pub mod bash_validation;
 mod bootstrap;
 mod compact;
 mod config;