CLI gen: tri-state nullable bool option parser (--flag / --flag false / absent)

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

Today

System.CommandLine's default Option<bool> is binary: presence means true, absence means false. There's no way to express "explicitly false" — which matters whenever a generated flag corresponds to a nullable: true boolean in the schema:

• null (absent) → don't override the loaded base body
• true → set field true
• false → set field false

Without tri-state, --only-main-content (which means "yes") can't be distinguished from "omit" vs. "explicitly disable".

Target

firecrawl scrape <url> --only-main-content              # → true
firecrawl scrape <url> --only-main-content false        # → false
firecrawl scrape <url>                                  # → null (don't override)
firecrawl scrape <url> --input base.json --only-main-content false   # → override base to false

Proposed approach

1. Emit Option<bool?> with custom parser for properties typed bool? (or bool with nullable: true):
   • parser accepts: nothing after the flag → true; true/yes/1 → true; false/no/0 → false; anything else → parse error.
2. Helper factory in CliRuntime: CreateNullableBoolOption(string name, string? description) to keep the per-property emission concise.
3. Help text: "true/false; presence alone implies true; omit to inherit".
4. Plain bool properties (non-nullable, default false) stay as Option<bool> — no behavior change.

Acceptance criteria

• Generated commands for nullable-bool body fields support all three states above.
• Snapshot test covers a spec with a nullable boolean property.
• Firecrawl scrape --only-main-content false works end-to-end after regen.

Start at nullable-bool option emission and the proposed CliRuntime helper, comparing them with the existing plain-bool path. Add or extend the snapshot test for a spec with a nullable boolean, then regenerate the Firecrawl scrape command and verify its end-to-end behavior. Done means absent, true, and false inputs preserve the three intended states.