refactor(test/cli): simplify pass on tier-A + prebuild

Applies findings from the third simplify pass:

1. \`cliArgv\` is a module-level const, not a function — prebuiltCli is
   read once at module init and never mutated, so the per-spawn function
   allocation was pure overhead.

2. Help-snapshot failures surface via \`Effect.fail\` instead of \`throw\`,
   symmetric with the \`Effect.fail\` already inside the partition above.
   Keeps the failure typed in the Effect channel rather than as a defect.

3. \`prebuild-test-cli.ts\` skips the build when the binary is already
   newer than every file in src/ — saves the 2.8s rebuild cost on every
   subsequent invocation. Pass --force to bypass.

4. \`prebuild-test-cli.ts\` verifies the built binary is executable before
   symlinking — catches a silently-failed build leaving stale output
   instead of letting tests fail with confusing exec errors later.

Verified: 331/331 CLI tests pass; typecheck clean; skip-if-fresh + --force
behave as documented.