fix(batch): export NODE_OPTIONS as a statement so compound shell commands work (#925)

[hakanbaysal]

What / Why / How

Fixes #925.

Batch shell commands that start with a compound construct (if, for,
while, a (...) subshell, or a { ...; } group) failed with
syntax error near unexpected token. The failed run was then indexed, so
ctx_search surfaced the syntax error instead of the intended output.

Root cause: the POSIX branch of buildBatchNodeOptionsPrefix emitted an inline
NODE_OPTIONS='...' <command> assignment. That form is only valid before a
simple command, so prefixing if .../for .../(...) produced invalid
shell.

Fix: emit a standalone export NODE_OPTIONS='...'; statement instead. This
mirrors the PowerShell ($env:...; ) and cmd (set "..." && ) branches, which
already use a separate statement, and composes with any following command. As a
bonus, the exported value now also reaches node invocations inside compound
constructs (e.g. a for loop body), which the inline form could not do.

Affected platforms

• All platforms

Test plan

• Real-shell verification: the old NODE_OPTIONS='...' if ...; then ...; fi
  errors at then; the new export NODE_OPTIONS='...'; if ... runs, as do
  for loops and (...) | sed subshell pipes. Confirmed export propagates
  NODE_OPTIONS to child node processes.
• Updated the POSIX prefix unit test and added coverage asserting the prefix
  composes with the three compound examples from the issue.
• npx vitest run tests/core/server.test.ts -t "buildBatchNodeOptionsPrefix"
  → 4 passed; runBatchCommands suite → 25 passed.
• npx tsc --noEmit → clean.

Checklist

• Tests added/updated (TDD: red → green)
• npm test passes (server suite; full build requires Node >=22.5)
• npm run typecheck passes
• Docs updated if needed — n/a
• No Windows path regressions (Windows/PowerShell/cmd branches unchanged)
• Targets next branch