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

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 with ./script/format.ts
- .gitignore: exclude bun-build files

Significantly reduces errors in the output in my experience.

Signed-off-by: Christian Stewart <[email protected]>

Author: paralin

.gitignore

@@ -18,3 +18,4 @@ Session.vim
 opencode.json
 a.out
 target
+*.bun-build

AGENTS.md

@@ -2,6 +2,34 @@
 
 - To test opencode in the `packages/opencode` directory you can run `bun dev`
 
+## Code Formatting
+
+- After changing TypeScript code, run `./script/format.ts` to format the code
+- This ensures consistent formatting across the codebase
+
+## Type Safety
+
+- NEVER use `as any` or type casts to work around type errors caused by outdated SDK types
+- If adding new schemas/types to `message-v2.ts` or other core files causes SDK type mismatches, regenerate the SDK first
+- Run `cd packages/sdk/js && bun ./script/build.ts` to regenerate SDK types after schema changes
+- Type errors are signals that something needs to be fixed properly, not suppressed
+
+### Type Predicates for Discriminated Unions
+
+- NEVER use `as` type assertions when filtering discriminated unions (e.g., `find()`, `filter()`)
+- ALWAYS use type predicate functions instead:
+
+```typescript
+// ❌ BAD: Using 'as' cast
+const subtask = parts.find((x) => x.type === "subtask") as SubtaskPart | undefined
+
+// ✅ GOOD: Using type predicate
+const isSubtaskPart = (part: Part): part is SubtaskPart => part.type === "subtask"
+const subtask = parts.find(isSubtaskPart)
+```
+
+**Why**: TypeScript's discriminated union narrowing doesn't work with array methods like `find()` or `filter()` because the predicate function doesn't act as a type guard. The return type is still the full union type, not the narrowed type. Type predicates (`part is Type`) explicitly tell TypeScript that the returned value matches a specific variant of the union.
+
 ## Tool Calling
 
 - ALWAYS USE PARALLEL TOOLS WHEN APPLICABLE. Here is an example illustrating how to execute 3 parallel file reads in this chat environment: