CLI gen: shared option set for request schemas reused across operations

Part of #338. Depends on gap #1 (per-parameter flags).

Today

Once gap #1 lands, each operation gets its own per-property flag list. For schemas reused across multiple operations (Firecrawl's ScrapeOptions is used by scrape, batch-scrape, and embedded in crawl), that produces:

• duplicated Option<T> field definitions across multiple .g.cs files (drift risk)
• no way to share a single --flag set under a prefix (--scrape-* in crawl)

Firecrawl's hand-written CliOptions.CreateScrapeOptionSet() returns a record with 28 options, used bare in scrape and prefixed --scrape-* in crawl. ~250 lines of careful manual wiring.

Target

When the generator detects a schema used as a request body or nested-in-request-body in 2+ operations:

1. Emit a single OptionSet static class with all the per-property Option<T> fields.
2. Each consuming command imports the option set bare or prefixed.

# bare (when ScrapeOptions IS the body)
firecrawl scrape <url> --only-main-content --formats markdown

# prefixed (when ScrapeOptions is nested inside CrawlRequest.scrapeOptions)
firecrawl crawl <url> --scrape-only-main-content --scrape-formats markdown

Proposed approach

1. Detect reuse: walk all operation request bodies (plus recursive into nested objects); any ModelData referenced 2+ times qualifies as an option set candidate.
2. Generate <ModelName>OptionSet class with one static Option<T?> per scalar/array/enum property and a Bind(parseResult) method returning the populated model.
3. Per-command consumption:
   • bare: command.Options.Add(ScrapeOptionsOptionSet.OnlyMainContent); ...
   • prefixed: a WithPrefix(\"scrape-\") factory that returns aliased Option<T> instances pointing at the same backing schema.
4. Vendor extension x-cli-option-set: false to opt out (always inline).
5. Collision rule: when both a top-level body property and a prefixed nested property would produce --max-depth, the top-level wins and the nested becomes --<prefix>-max-depth.

Acceptance criteria

• Regenerating Firecrawl produces one ScrapeOptionsOptionSet.g.cs shared between scrape and crawl.
• firecrawl crawl <url> --scrape-only-main-content --scrape-formats markdown works.
• No duplicated Option<T> field definitions across .g.cs files for the same schema property.

Start by tracing how request bodies and nested objects become generated .g.cs files, using Firecrawl's hand-written CliOptions.CreateScrapeOptionSet() as the existing behavior reference. Check the generated output for scrape and crawl; done means one shared ScrapeOptionsOptionSet.g.cs, working prefixed flags, and no duplicated Option fields for the same schema property.