refactor: converge the process-group kill primitive into util/process

[LeXwDeX]

Why

The platform group-kill primitive (win32 taskkill /T /F vs unix negative-pid process.kill) is duplicated between hook/settings.ts execShell (its killGroup) and util/process.ts (stop win32 branch, signalTree), with diverging error handling. Code review of #500-504 flagged the shape as drifting (four near-copies of the escalation chain; the SDK copy has already diverged semantically).

Scope

• packages/opencode/src/util/process.ts, packages/opencode/src/hook/settings.ts (+ their tests).
• OUT of scope: packages/sdk/js/src/process.ts (published-package structural copy, documented and accepted in fix: unify child-process stop semantics with await-exit and SIGKILL escalation #503).

Approach

• Extract a shared killGroupPid(pid, signal?) primitive in util/process.ts: unix process.kill(-pid, signal), win32 taskkill /pid <pid> /T /F, best-effort with a boolean result; reuse it in stop's win32 branch and replace the hook's killGroup body with it (hook keeps its own logging and killSent state).
• No behavior change to the fix: hook command grandchildren hold stdio pipes and hang triggers forever #500 timeout semantics: same signals, same ordering, same grace constants.

Acceptance

• hook/ and util/process test suites green including the fix: hook command grandchildren hold stdio pipes and hang triggers forever #500 process-group regression tests; platform kill branching has a single source inside the opencode package; typecheck and lint unaffected.

[LeXwDeX]

SpecGit delivery branch: test/512-leftover-hardening (draft pull request #516).

[LeXwDeX]

Delivered by PR #516 (merged into dev at 94f5471). GitHub only fires Closes references on PRs targeting the default branch, so this is closed manually; the sync PR to main carries the delivery onward.