Today 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. As a result, embedders that want to
forward Kit's low-level engine + MCP tool plumbing debug output into
their own logging system (slog, zap, charm/log, an in-app TUI panel,
etc.) have no option but the on/off Debug bool, which always installs
the built-in SimpleDebugLogger / BufferedDebugLogger.

This change closes that gap on the supported Options / functional-option
construction path:

- 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.

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.

kit.AgentConfig (pkg/kit/types.go) and its toInternal converter were
exposed as the documented "low-level / advanced consumer" path for
agent construction, but the converter was unexported and not wired
into any public constructor — neither New(*Options) nor NewAgent(...Option)
accept an AgentConfig. The only call sites were the dedicated
agent_config_internal_test.go (same-package internal test) and two
fantasy-import regression tests in types_test.go.

Net effect today: no SDK consumer outside pkg/kit can populate or use
kit.AgentConfig in any way. The type, the converter, the dedicated
test file, and a chain of godoc cross-references all exist purely for
their own sake — they don't enlarge what consumers can do, but they
do enlarge the SDK's stability contract (every field becomes a public
shape the internal agent layer can't refactor freely).

The companion PR added Options.DebugLogger + WithDebugLogger so the
last functional capability AgentConfig was documented to enable —
installing a custom debug logger — is reachable through the supported
construction path. With that wired, AgentConfig has no remaining
purpose.

Changes:

- pkg/kit/types.go: remove the AgentConfig struct and its toInternal()
  method. Drop the now-unused internal/agent and internal/tools
  imports. Update the DebugLogger godoc to point at Options.DebugLogger
  and WithDebugLogger instead of AgentConfig.
- pkg/kit/agent_config_internal_test.go: deleted (208 LOC). It exercised
  the unexported toInternal() method directly; with the method gone
  the test has no subject.
- pkg/kit/types_test.go: rename TestAgentConfigNoFantasyImport to
  TestOptionsNoFantasyImport and rewrite it against Options
  (SystemPrompt, MaxSteps, Streaming, Tools, ExtraTools,
  DisableCoreTools, OnMCPServerLoaded). The original test also asserted
  ToolWrapper field semantics; that capability migrates to the hook
  system (OnBeforeToolCall / OnAfterToolResult), already covered by
  hooks_test.go, so the assertion is dropped with a pointer in the
  godoc. TestAgentConfigToolWrapperSignature replaced by
  TestToolSliceSignature, which still pins that []kit.Tool is the
  user-visible slice type for every tool-related SDK surface — the
  no-fantasy-import contract the original test guarded.
- pkg/kit/mcp_tasks.go: update the MCPTaskConfig godoc to stop
  referencing AgentConfig. MCPTaskConfig stays — it is still emitted
  through Options.MCPTask* fields and used as the engine-facing
  config type.
- pkg/kit/README.md: drop the kit.AgentConfig line from the type
  inventory.

internal/agent.AgentConfig is untouched and remains the internal
construction shape. With the public type gone the internal one can
evolve freely without breaking the SDK contract.

Verification:
- go build ./pkg/... ./internal/... ./cmd/... — clean
- go vet ./pkg/... ./internal/... ./cmd/... — clean
- go test -race -timeout 300s ./... — all packages pass

- README.md: add WithDebugLogger to the functional-options helper list
- pkg/kit/README.md: expand the Debug row and add a DebugLogger row in
  the Options field summary
- www/pages/sdk/overview.md: add WithDebugLogger to the helpers table
  with a note that it overrides WithDebug when set
- www/pages/sdk/options.md: surface DebugLogger in the example, expand
  the Debug field description, add a DebugLogger row to the Core
  fields table, and add a "Custom debug logger" section with the
  interface signature and a log/slog adapter example