Fix documentation errors for Custom Prompts named arguments and add canonical examples

[Sayeem3051]

The Custom Prompts documentation (docs/prompts.md) was incomplete for named arguments:

1. Documentation for custom prompts was incomplete - named argument usage was mentioned briefly but lacked comprehensive canonical examples showing proper syntax and behavior.

2. Fixed by adding canonical, tested syntax and examples:

   • Example 1: Basic named arguments with TICKET_ID and TICKET_TITLE
   • Example 2: Mixed positional and named arguments with FILE and FOCUS
   • Example 3: Using positional arguments
   • Example 4: Updated draftpr example to use proper $FEATURE_NAME syntax
   • Added clear usage examples showing KEY=value syntax
   • Added expanded prompt examples showing the result
   • Documented error handling and validation requirements

3. Added Implementation Reference section that references the relevant feature implementation from the codebase (PRs Custom prompt args (numeric) #4470 and Named args for custom prompts #4474 for initial implementation, fix: improve custom prompt documentation and actually use prompt descriptions #5332 and docs: clarify prompt metadata behavior #5403 for clarifications).

This addresses issue #5039 by providing complete, accurate documentation for named argument usage in custom prompts.

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[Sayeem3051]

I have read the CLA Document and I hereby sign the CLA

[Sayeem3051]

recheck

[etraut-openai]

Thanks for the contribution. I think there is a need for more documentation here. Here are a few suggestions:

1. You've included four examples. That's a bit much. Could you pare it down to two examples — perhaps a simple one and a more advanced one?
2. I don't think the "Implementation reference" section belongs in the public docs. Recommend deleting.