perf(test): reduce CLI E2E startup overhead

[XXPermanentXX]

Summary

• render registered command and group help in-process with the real CommandRegistry
• migrate ordinary command --help assertions to the shared in-process helper while retaining representative subprocess smoke coverage
• run remaining TypeScript subprocess entrypoints with node --import tsx to avoid the tsx CLI IPC startup
• add helper/runner contract tests and update the E2E maintenance guide

No production CLI behavior is changed.

Performance

Local runs with two Vitest workers:

• command help subset: 63.70s → 15.58s (about 75% faster)
• dry-run subset: 95.89s → 84.27s in the isolated profiling run (about 12% faster)

GitHub Actions remains the authoritative end-to-end timing.

Verification

• vp check — 0 errors; 5 pre-existing warnings
• affected help/registry/runner tests — 337 passed, 560 skipped
• vp run -r build — passed outside the managed sandbox
• git diff --check main...HEAD — passed

A broad local -t dry-run run inherited developer Profile/.env state and reported unrelated default-value/network/local-listener failures (187 passed, 6 failed). The live API suite was not used as a completion gate to avoid external calls; CI should validate the clean-environment test matrix.