fix(cli): add --limit to the enhance command across all three layers

[gadievron]

The enhance command lacked a --limit (max-units) capability while its siblings analyze and scan
have it, so a user could not cost-limit or test-run enhancement on N units -- enhance always processed the
full dataset. The gap spanned all three layers (a Go-only patch would be inert: the Python CLI rejected
--limit and the core could not consume it):

• Go (apps/openant-cli/cmd/enhance.go): add a --limit flag (default 0 = no limit) and forward it to the
  Python args when > 0, mirroring analyze.go.
• Python CLI (libs/openant-core/openant/cli.py): add --limit to the enhance subparser and pass
  limit=args.limit into enhance_dataset, mirroring analyze.
• Core (libs/openant-core/core/enhancer.py): add a limit parameter to enhance_dataset and slice
  dataset["units"] to units[:limit] before enhancement, mirroring analyzer.py (limit + units[:limit]).
  Slicing dataset["units"] (not only the local list) is what makes the limit reach both the agentic and
  single-shot enhancement paths.

Scope: analyze.go / analyze_p / analyzer.py already have --limit (the parity baseline, unchanged here);
verify/report/etc. do not enhance units, so they are out of scope.

Tests:

• libs/openant-core/tests/test_enhance_limit.py: with the LLM stubbed, enhance_dataset(limit=2) on a
  5-unit dataset makes the enhancer RECEIVE exactly 2 units (asserting on what the enhancer receives guards
  the load-bearing dataset["units"] slice -- verified by a mutant: dropping that line makes the enhancer
  see all 5 and the test fail); and enhance ... --limit 3 driven through cli.main() forwards limit=3 to
  enhance_dataset (covers the enhance_p parser + cmd_enhance glue).
• apps/openant-cli/cmd/enhance_limit_test.go: enhanceCmd exposes a --limit flag (default 0).
  RED (pre-fix, via stash): Python "unrecognized arguments: --limit" + Go "missing --limit flag" -> GREEN.
  Full py suite 178 passed / 63 skipped; go test ./cmd/ ok; ruff + gofmt + go vet clean.

Co-Authored-By: Claude Opus 4.7 (1M context) noreply@anthropic.com

[gadievron]

Closing as already resolved on current master. This fix's behavior shipped via the parser-fix-stack release (#133/#134); confirmed by independent reproduction — the original bug (reconstructed from its bug-pipeline investigation) no longer manifests on master, not merely by line-presence. Part of a full validated sweep of the open-PR corpus.