fix(agent): reap agent process group on normal exit, not just cancellation

[Christopher-Li]

Problem

shellenv.ConfigureShellCommand isolates each agent/command in its own process group (Setpgid) and installs cmd.Cancel to SIGKILL the group - but cmd.Cancel only fires on context cancellation. The success path is never covered.

When the test step's agent (or a configured test/lint command) exits 0, nothing kills the group. Grandchildren that outlived the leader - a vitest tinypool worker pool, a build watcher, a dev server - reparent to init (PPID 1) and keep running. A clean vitest run leaks its worker pool every time, which is the observed RAM-exhaustion (17×~1 GB orphan workers).

So cancellation was handled; clean completion was not.

Fix

• Add shellenv.TerminateShellCommandGroup across the build-tag split:
  • unix: SIGKILL of -pgid (the group persists while any member is alive, so it still reaps grandchildren that outlived the leader)
  • windows: best-effort taskkill /T /F /PID
  • other: no-op stub
• defer shellenv.TerminateShellCommandGroup(cmd) right after Start() in every native agent runner (claude, pi, acpx, codex) and the shell-command path (runShellCommandWithEnv). The defer covers success, parse-error, and wait-error returns; cmd.Cancel still covers cancellation, where the defer is a harmless ESRCH no-op.
• Install a 5s WaitDelay backstop on unix (mirroring the existing windows backstop) so a grandchild holding an inherited stdout/stderr pipe open can't wedge Wait forever.

This fixes the general class, not just vitest: any agent-spawned grandchild leaking after a clean step is now reaped.

Tests

• internal/shellenv: TerminateShellCommandGroup reaps a backgrounded grandchild left by a normally-exited leader; nil/never-started cmd is a no-op.
• internal/agent: drives claudeAgent.runOnce against a fake claude that backgrounds a grandchild and exits 0; asserts the grandchild is gone after runOnce returns. Confirmed this test fails without the defer (grandchild survives) and passes with it.

Verification

gofmt clean · make lint · go test -race ./... · make e2e · Windows + wasip1 cross-builds all pass.