docs: add Go coding policies to AGENTS.md per review request

AGENTS.md

@@ -141,3 +141,17 @@ Version: 2025-06-08
    - Local application imports
    
    Each group should be separated by a blank line.
+
+## Go-Specific Guidelines
+
+1. **No `panic`, `log.Fatal`, or `os.Exit` in library code.** Always propagate errors via return values.
+
+2. **Constructors return `(*T, error)`, not just `*T`.** Callers must handle errors, not crash.
+
+3. **Wrap errors** with `fmt.Errorf("context: %w", err)` for debuggability.
+
+4. **Never modify linter config** (`.golangci.yml`) to suppress findings. Fix the code.
+
+5. **All PRs must pass `make check` with zero failures.** No exceptions, no "pre-existing issue" excuses.
+
+6. **Pin external dependencies by commit hash**, not mutable tags.