feat(shell): support file redirects in run_command

[esengine]

Phase 2 of #232. Builds on #233 + #234.

What changes

run_command now parses and executes file redirects natively. Each segment of a chain (or a standalone command) can carry trailing redirects; the parser reads them, the runner opens the files with fs.openSync and hands the fds to spawn's stdio. No shell is invoked — semantics are identical on Windows / macOS / Linux.

Op	Meaning
>	stdout to file (truncate)
>>	stdout to file (append)
<	stdin from file
2>	stderr to file (truncate)
2>>	stderr to file (append)
2>&1	merge stderr → wherever stdout points
&>	both stdout + stderr to file (truncate)

Targets resolve relative to the chain's cwd. At most one redirect per fd per segment — multiple >s in one segment are rejected with a clear conflict message.

Examples

echo hi > out.txt                           # truncate-write
echo more >> log.txt                        # append
sort < data.txt | uniq                      # stdin from file, pipe to next
node script.js > out 2>&1                   # capture both streams to one file
node script.js &> all.log                   # same, shorthand
node script.js 2> errors.log                # split stderr off from stdout
git diff > /tmp/p.patch && git apply /tmp/p.patch

Why this didn't ship in #233

shell-quote's POSIX strictness conflicted with the project's lenient tokenizer (#234 fixed that by going back to whitespace-bounded chain detection). With the parser layer now consistent with the project's existing primitives, redirects fit cleanly: same single-pass walker, same quote awareness, same lenient handling of embedded &/|/; chars.

Implementation

ChainSegment now carries redirects: Redirect[] alongside argv. New parseSegment(segStr) is a single-pass walker that emits both argv tokens and redirect ops at the same level. Old per-segment tokenizeCommand + detectShellOperator combo is gone — replaced by the unified parser.

runPipeGroup opens redirect fds per segment via fs.openSync (always closed in finally), then hands them to spawn's stdio. Pipe wiring still applies: < overrides pipe-in, >/>>/&> override pipe-out / capture-buffer. 2>&1 is implemented by setting stderr's stdio spec to whatever stdout's points at — works for buffer-capture, fd, and pipe equally.

Compat notes

• Background & still rejected — use run_background.
• Heredoc << still rejected — use < file or the binary's flag.
• \$VAR and \$(...) pass through as literal characters (matches the simple-command tokenizer's pre-existing behavior).
• The lenient embedded-&/|/; behavior from fix(shell): preserve lenient tokenization in chain segments #234 is preserved (cargo run -- --flag=1&2 still works).

Tests

• tests/shell-redirects.test.ts — 30 new tests covering parser output for every redirect kind, stuck-to-target form, quoted targets, conflict detection, and execution (truncate/append, stdin-from-file, stderr split, 2>&1 merge, &> both, redirects across pipe boundaries).
• tests/shell-tools.test.ts — old "rejects stdout redirect / stderr-merge" tests rotated out; added rejections for missing target, heredoc, multi-redirect-per-fd.
• tests/shell-chain.test.ts — old "rejects redirects inside a chain segment" test repurposed to assert successful parsing.
• npm run verify — 2228 tests pass, no biome / typecheck errors on changed files.