feat: workflow_intent Go SDK support

[sftimeless]

Summary

• Adds the Go SDK workflow_intent surface: client.Workflows.Declare/Amend/Complete/Get/List, workflow JSON types, README docs, and a compile-tested examples/workflows sample.
• Uses Go-native context.Context propagation: WithWorkflow, WorkflowFromContext, and RunInWorkflow, with httpTransport.newRequest auto-injecting X-Keel-Workflow-Id only when the supplied context carries a workflow ID.
• 8 commits (e8dd0eb types -> 35e093b gofmt). The final gofmt commit is mechanical because origin/main already had formatting drift and the requested gate is gofmt -l . empty.

Highlights

• Mirrors the workflow declaration response shapes from the API schema, with Go time.Time values for API datetimes and string UUIDs to keep the SDK dependency-free.
• Adds workflow reason-code sentinels so callers can use errors.Is(err, keel.ErrWorkflowMaxCallsExceeded) while preserving *KeelError / *ThrottledError handling.
• Locks workflow header behavior with tests: context-scoped requests get the header, requests outside the context do not, goroutines keep the workflow ID when passed the same context, and proxy payloads do not receive the internal workflow header.
• Adds README usage for the WithWorkflow + HTTP auto-injection pattern and a full declare -> permit calls -> amend -> complete example.

Test plan

• Baseline origin/main: go test ./... passed before changes (22 top-level test functions)
• Branch go build ./...: passed
• Branch go test ./...: passed (32 top-level test functions, 22 -> 32)
• Branch gofmt -l .: clean
• Branch go vet ./...: passed

Judgment calls

• RunInWorkflow accepts func(context.Context) error instead of using ambient global state. This keeps propagation explicit and idiomatic for Go.
• The SDK exposes WorkflowAmendRequest as requested while targeting POST /v1/workflows/{workflow_id}/amend, matching the workflow design and the JS SDK worktree.
• The prompt's exact keel-python sdk/python/workflows.py path was not present locally; API schemas, the workflow design doc, and the JS workflow SDK worktree were used as the concrete surface references.

Out of scope

• Provider SDK middleware patching
• LangChain Go bindings
• gRPC variants
• Streaming workflows