fix(vtz): vtz ci self-hosts config loading — drop bun/tsx requirement

[viniciusdacal]

Summary

Resolves #2739. vtz ci's config loader used to spawn an external JS runtime to evaluate ci.config.ts — bun preferred, node --import tsx as fallback. That meant bun (or tsx as a devDep) was a hard requirement for vtz ci despite vtz itself being a TypeScript runtime.

Surfaced during the bun→vtz CI migration attempt (closed PR #2733): without bun on PATH, the node+tsx fallback also failed because @vertz/ci's package.json exports don't satisfy strict-Node ESM resolution, which tsx uses. Piling workarounds would have masked the real architectural problem.

This PR makes vtz self-hosting for this path — with no fallback. If the running vtz binary path can't be determined, vtz ci fails fast with a clear error rather than silently picking up bun or tsx (which would mask real bugs in the vtz install / execution path).

Changes

1. vtz __exec <file> [args...] — new hidden subcommand

Runs a single JS/TS file through the vtz runtime with process.argv populated as [vtz_bin, abs_file, ...extra_args]. Not for end-user use — it exists to support internal tooling like vtz ci.

2. find_runtime() in ci/config.rs

Always uses std::env::current_exe() + the __exec subcommand. No bun, no tsx, no fallbacks. If current_exe() fails (extremely rare — platform-specific sandbox edge cases), vtz ci surfaces the error instead of silently switching interpreters.

3. process.exit(code) is now implemented

Via new op_process_exit op. It previously threw. The existing .pipe/_loader.mjs (unchanged in this PR) calls process.exit(0) at the end of its run, so this is necessary for the loader to terminate cleanly under vtz. Flushes stdout/stderr before exit.

Verified end-to-end locally

$ vtz ci test --scope @vertz/errors --concurrency 1
[pipe] Loading ci.config.ts...           ← loaded via vtz, not bun/tsx
[pipe] Workspace: 47 packages, 3 native crates
[pipe] Running with concurrency=1
...

No bun on PATH, no tsx devDependency — and vtz ci works.

Rust quality gates

• cargo test -p vtz --lib — 3441 pass, 0 fail
• cargo clippy -p vtz --all-targets -- -D warnings clean
• cargo fmt --all -- --check clean

Unblocks

After this lands + 0.2.69 publishes, the bun→vtz CI migration (formerly PR #2733) becomes a trivial workflow-only change.

Related tracking issues

• fix(vtz): preserve executable mode when vtz install extracts tarballs #2735 — tarball extraction exec mode (fixed + shipped in 0.2.68)
• vtz install resolver: ^0.27.3 incorrectly resolves to 0.25.12 #2738 — vtz install semver resolver bug (open)
• vtz ci: load ci.config.ts via vtz runtime itself (not bun/tsx) #2739 — this PR

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