move SDK to pkg/kit, extract shared logic from cmd, relocate main to cmd/kit

Restructure the codebase so the CLI app consumes the SDK rather than
the SDK wrapping CLI internals. This eliminates the circular dependency
(sdk -> cmd -> sdk) and establishes pkg/kit as the canonical API.

Key changes:
- Create pkg/kit/ with InitConfig, SetupAgent, BuildProviderConfig
  extracted from cmd/root.go and cmd/setup.go as parameterized functions
- Move sdk/kit.go -> pkg/kit/kit.go (remove cmd import, use local calls)
- Move sdk/types.go -> pkg/kit/types.go
- Move main.go -> cmd/kit/main.go (standard Go project layout)
- cmd/root.go and cmd/setup.go now delegate to pkg/kit, injecting
  CLI-specific state (quietFlag) via the Quiet field on AgentSetupOptions
- Add setSDKDefaults() for cobra-free SDK usage (viper defaults)
- Fix .gitignore: kit -> /kit (was blocking cmd/kit/ and pkg/kit/)
- Update .goreleaser.yaml, Taskfile.yml, AGENTS.md, contribute/build.sh,
  README.md for new cmd/kit entrypoint and pkg/kit import paths
- Add plans/ with 10 detailed SDK revamp plans and Taskfile.yml
- Delete sdk/ directory entirely

AGENTS.md

@@ -20,7 +20,7 @@ Keep this managed block so 'openspec update' can refresh the instructions.
 # KIT Agent Guidelines
 
 ## Build/Test Commands
-- **Build**: `go build -o output/kit`
+- **Build**: `go build -o output/kit ./cmd/kit`
 - **Test all**: `go test -race ./...`
 - **Test single**: `go test -race ./cmd -run TestScriptExecution`
 - **Lint**: `go vet ./...`