fix(shell): preserve output from fast-exiting commands

[kitlangton]

Why

Short-lived shell commands can finish successfully but return (no output) instead of the bytes they printed. Bun resumes child stdout and stderr on exit; our Effect readers attach lazily, so output can be discarded before consumption starts.

The failure also reproduces using the spawner implementation from before #46085. This is an older capture bug, not a reason to remove the post-exit drain deadline.

What Changes

• Pipe stdout and stderr into backpressured PassThrough buffers when setting up the handle, before the lazy Effect readers attach. This follows the capture pattern in Effect's Node spawner.
• Let the stream adapter clean up the capture buffer while preserving the existing raw-stream cleanup policy.
• Unpipe before deadline-driven discard, so late descendant output cannot refill an abandoned capture buffer.

flowchart LR
  Child[Child stdout / stderr] --> Buffer[Eager PassThrough buffer]
  Buffer --> Reader[Lazy Effect reader]

Loading

The regression tests keep the process scope open, wait for a real child to exit, then read its output. Before the fix, separate and combined capture both return empty output in under 60 ms. Afterward, both retain stdout and stderr. A third case checks simultaneous 256 KiB outputs across backpressure.

Scope

Only child-output capture and its tests change. The shared one-second post-exit deadline, process-group termination, SIGKILL escalation, and MCP shutdown behavior remain intact. No public API or SDK cancellation-contract changes.

Verification

Run from package directories on macOS with Bun 1.3.14:

cd packages/core
bun typecheck
bun run test test/effect/cross-spawn-spawner.test.ts test/tool-shell.test.ts test/session-shell.test.ts test/shell-cleanup.test.ts test/mcp.test.ts
bun run test test/effect/cross-spawn-spawner.test.ts --test-name-pattern 'delayed output consumption|captures stdout via' --rerun-each 100
bun run test test/tool-shell.test.ts --test-name-pattern 'ordinary shell syntax' --rerun-each 10
bun run test

cd ../util
bun typecheck
bun run test
bun run build

• Targeted spawner, shell, Session-shell, cleanup, and MCP suites: 168 pass, 19 skip, 0 fail.
• Repeated capture checks: 300 pass, 0 fail. Repeated shell-syntax checks: 320 pass, 190 skip, 0 fail.
• A separate real-process probe ran 500 commands at concurrency 16 and retained both stdout and stderr in every case.
• The built Util package also retained output in 100 delayed-consumption checks under Node 26.5.0 at concurrency 8.
• Util tests: 27 pass, 0 fail. Util builds, Core and Util typechecks, and all 33 repository pre-push typecheck tasks pass.
• Prettier and git diff --check pass. Targeted lint has no errors and three pre-existing warnings.
• Full Core suite: 4,047 pass, 40 skip, 0 fail across 229 files, including the formerly intermittent shell-output failures.