docs(agents): add guidance to make working with opencode better

[paralin]

After working with the codebase using opencode a bit:

• Add guidance on regenerating SDK after schema changes
• Document type predicate pattern for filtering discriminated unions
• Discourage use of 'as' casts when TypeScript's type narrowing fails
• Add guidance to run prettier after changing code
• .gitignore: exclude bun-build files

Significantly reduces errors in the output in my experience.

[paralin]

hey @jerome-benoit I'm still having good success w/ this on my dev branch. did you want any more changes / improvements?

[jerome-benoit]

hey @jerome-benoit I'm still having good success w/ this on my dev branch. did you want any more changes / improvements?

No, thank you. You should ping someone in the opencode team to recall him/her that PR.