WasmVM brush-shell: `>>` append truncates and `< file` stdin redirection fails

Summary

Two shell-redirection behaviors are broken in the WasmVM guest shell
(brush-shell), surfaced by the runtime-core integration suite:

1. Append redirection (>>) truncates instead of appending.
2. Stdin redirection (cmd < file) fails with a non-zero exit.

Both diverge from Linux/POSIX shell semantics.

Failing tests

packages/runtime-core/tests/integration/bridge-child-process.test.ts

Exact assertion output:

× execSync append redirection onto a write-only file succeeds like Linux
  → expected 'changed' to be 'originalchanged' // Object.is equality

× execSync stdin redirection feeds the kernel VFS file to the command
  → stdout:
    Error: Command failed: cat < stdin-input.txt
    : expected 1 to be +0 // Object.is equality

Repro

# build/vendor the WASM commands, then:
pnpm --filter @rivet-dev/agentos-runtime-core exec vitest run \
  tests/integration/bridge-child-process.test.ts

The tests route child_process.execSync(...) through the bridge → kernel →
WasmVM sh (brush-shell), so the failures are in the guest shell's handling of
>> and <, not in the Node bridge.

Suspected root cause

• >>: the append open-mode is not honored against the kernel VFS — the
  file is opened truncating (O_WRONLY|O_TRUNC) instead of appending
  (O_WRONLY|O_APPEND). Related patched area:
  toolchain/std-patches/crates/brush-* (WASI path/redirection support).
• < file: input redirection from a VFS path is not wired to the command's
  stdin, so cat reads nothing and exits non-zero.

Not a regression

These predate and are unrelated to the registry→software flatten/colocation
refactor. Verified: the failing test file is unchanged, the vendored
runtime-core/commands binaries the test runs are unchanged, the brush-shell
source patches only moved (0 content delta), and the only runtime-core source
change is a moot commands-dir path string. The failures reproduce identically on
the parent revision.

Impact

Any guest program relying on >> append or < stdin redirection behaves
incorrectly (silent data loss on append; command failure on stdin redirect).

Related (separate issue)

The same integration run also has a WasmVM signal/dispose cluster — SIGKILL/
SIGTERM not terminating guest processes and dispose timing out
(signal-forwarding.test.ts, dispose-behavior.test.ts). Distinct root cause
(process signal delivery, not shell redirection); should be tracked separately.

Start by running packages/runtime-core/tests/integration/bridge-child-process.test.ts with the provided pnpm command and inspect the failing append and stdin-redirection cases. Then read the related toolchain/std-patches/crates/brush-* area for WASI path and redirection support. Done means append preserves existing content and cat < stdin-input.txt exits successfully with the VFS file contents.