Summary

The SDK exposes a DebugLogger interface (pkg/kit/types.go) but no public path to install one. The only consumer of the field is the unexported kit.AgentConfig.toInternal() method, which itself is not reachable from outside the package. Embedders that want to forward Kit's low-level engine + MCP debug output into their own logging system (slog, zap, charm/log, an in-app TUI panel, etc.) currently have no option but the on/off Debug bool, which always installs the built-in SimpleDebugLogger / BufferedDebugLogger.

This PR closes that gap on the supported Options / functional-option construction path.

Changes

• pkg/kit/kit.go: add Options.DebugLogger DebugLogger. When non-nil it is used directly and the Debug bool is ignored; the supplied logger's IsDebugEnabled() controls whether downstream code emits messages.
• pkg/kit/options.go: add WithDebugLogger(l DebugLogger) Option.
• internal/kitsetup/setup.go: add AgentSetupOptions.DebugLogger and switch SetupAgent's logger selection so the caller-supplied logger wins unconditionally; otherwise the existing Debug + UseBufferedLogger branch picks the built-in implementation. No behaviour change when DebugLogger is nil.
• pkg/kit/kit.go: wire opts.DebugLogger into setupOpts so the New() path threads it through.
• pkg/kit/viper_isolation_test.go: add TestWithDebugLoggerPlumbing and TestWithDebugLoggerNilClears covering the option-to-field contract and later-options-override semantics consistent with the other With* helpers.
• pkg/kit/README.md: list WithDebugLogger in the helper inventory.

Usage

type myLogger struct{ /* slog/zap/charm-log wrapper */ }

func (l *myLogger) LogDebug(msg string)    { /* forward */ }
func (l *myLogger) IsDebugEnabled() bool   { return true }

k, err := kit.NewAgent(ctx,
    kit.WithModel("anthropic/claude-sonnet-4-5-20250929"),
    kit.WithDebugLogger(&myLogger{}),
)

Notes

• kit.DebugLogger and tools.DebugLogger are structurally identical (LogDebug(string) / IsDebugEnabled() bool), so the SDK value flows into the internal field without a conversion.
• This is purely additive on the SDK surface and does not touch kit.AgentConfig — that field already carried a DebugLogger, but the AgentConfig path is unreachable from outside the package today.
• A discussion thread suggested removing the unreachable kit.AgentConfig entirely; this PR is intentionally scoped to the "keep DebugLogger, add the missing wiring" option only. Removal of AgentConfig (if desired) belongs in a separate change.

Verification

• go build ./pkg/... ./internal/... ./cmd/... — clean
• go vet ./... — clean (modulo the pre-existing examples/extensions Yaegi source not being a main package)
• go test -race ./pkg/kit/... ./internal/kitsetup/... ./internal/agent/... ./internal/tools/... — passing

Summary by CodeRabbit

Release Notes

• New Features
  • Introduced WithDebugLogger option to supply custom debug logger implementations, allowing users to route and control debug output
  • Custom-supplied loggers override built-in debug logging when configured, providing full control over debug output handling
  • Debug state reporting now determined by the configured logger instance