[li-xiu-qi]

Fixes #276

Problem

When step update runs from the TUI and the installer script exits non-zero,
the only error shown is install.ps1 exited with code 1 — no installer
output, so users cannot see why the installer failed and cannot file an
actionable report.

Two things cause this in packages/coding-agent/src/step/local-update.ts:

• installLatestStepRelease builds the failure message from the exit code
  alone; the installer's stdout/stderr are never captured.
• runCommand spawns the installer with stdio: "inherit", but the TUI
  renders full-screen, so the child's output is overwritten before the user
  can read it.

Changes

• runCommand now pipes and captures the child's stdout/stderr instead of
  inheriting them, returning { code, output }.
• New formatInstallerFailure(scriptName, code, output) builds the failure
  message with the tail of the captured output (last 20 lines, noting how
  many earlier lines were omitted).
• installLatestStepRelease uses it for the non-zero-exit message.

Behavior and compatibility

• runCommand is module-private with a single caller
  (installLatestStepRelease); no other call sites.
• No changes to public interfaces, configuration keys, CLI arguments,
  persisted sessions, or provider responses.
• Installer output is no longer passed through to the parent terminal. This
  function only runs in the TUI update path (maybeUpdateStep's
  installUpdate default), where inherited output was already invisible
  under the full-screen render — so there is no visible-output regression.
  The CLI step update command takes the archive path
  (runStepUpdateCommand) and does not go through this function.
• The success message is unchanged (Updated Step in …).

Validation

• New tests in local-update.test.ts: three for formatInstallerFailure
  (no output / short output / long-output tail truncation) and one
  end-to-end test that mocks the installer download with a failing script,
  spawns it for real, and asserts the failure message contains the
  installer's output.
• vitest run src/step/local-update.test.ts — 8/8 pass (4 pre-existing
  findArchiveRoot tests + 4 new).
• A standalone script driving installLatestStepRelease with a mocked
  failing installer (real powershell/bash spawn) confirms the message
  content: the fixed code embeds the installer output, the pre-fix code
  shows the bare exit code only.
• npm run check — all green (biome, tsgo with zero errors, repo check
  scripts).
• Verified on Windows 11 x64.

Limitations

• Installer output on success is no longer echoed to the terminal. Under
  the TUI it was never visible; if a future CLI path reuses this function,
  pass-through would need revisiting.