Extend CLI batch execution

[Widthdom]

Summary

• Accept validated structured batch records ({"command":"...","args":[...]}) alongside the existing string-array form.
• Add configurable, safely bounded input-line and serialized-output budgets.
• Add opt-in parallel read-only execution with isolated DB connections and stdout/stderr capture, per-item failure isolation, stable input-order records, and incremental output before stdin EOF.
• Propagate caller cancellation through supported batch aliases and preserve input-order exit-code selection.

Root cause

The batch runner only accepted string arrays, used fixed workload budgets, and executed every item serially. Its original Console capture model was process-global, so it could not safely support concurrent child commands. The first parallel implementation also filled an entire worker window before starting work, which could deadlock request/response stdin producers.

Impact

Existing serial behavior remains the default. Parallel execution is limited to the read-only batch allowlist, requires --json-summary, uses bounded concurrency (maximum 16), and emits completed records in input order without waiting for a full window or EOF. Malformed records and command failures remain isolated and contribute to the final summary/exit status.

Validation

• make lint
• dotnet build CodeIndex.sln -c Release --no-restore (0 warnings, 0 errors)
• dotnet run --project tools/CodeIndex.Changelog -- check
• Release affected-scope tests after merging current origin/main: 332/332 passed on net8.0 and 332/332 passed on net9.0
• Issue-specific streaming/cancellation/budget/ordering tests: 6/6 passed on both net8.0 and net9.0
• Real-process incremental stdin smoke test confirmed the first result is emitted before EOF and the final summary follows after EOF
• cdidx status --check --json reported a fresh matching index with no failed checks
• Codex adversarial review: two rounds completed; all reported findings (three P2 and one P1) were addressed

A full solution test attempt earlier in the task did not complete in the shared host: the dual-TFM run ended with substrate exit 143 after 26 minutes without a reported failure, and the standard net8.0 run hit the unrelated existing Issue4592 watch-readiness timeout under heavy load. That test passed 1/1 when rerun in isolation; the final affected scope above is green on both TFMs.

Documentation and changelog

• Updated the English and Japanese batch documentation in USER_GUIDE.md, plus the matching developer/testing guidance.
• Added bilingual fragment changelog.d/unreleased/4723.added.md.
• Did not modify CHANGELOG.md, AGENTS.md, CLAUDE.md, or AGENT_GUIDE.md.

Fixes #4723