feat: foundations — canonical types, Parse grammar, env DSNs, health, chains

Phase 1 of the majordomo build:
- llm/ canonical contract (messages, parts, tools, capabilities, streaming,
  Model/Provider, error classification)
- health/ clock-injected tracker (threshold bench, exponential capped
  cooldown, reset-on-success)
- root Registry + Parse (verbatim model ids, inline recursive alias
  expansion with cycle detection, chain dedup), LLM_* env-DSN providers
  (go-llm parity: lazy fallback + eager LoadEnv), health-aware chain
  executor behind the Model interface
- provider/fake scriptable test provider; hermetic test suite incl. the
  trailing-thinking chain and foreman:// env loading
- ADRs 0001-0008, CLAUDE.md, README (honest matrix), CI workflow,
  docs/phase-1-design.md

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

llm/stream.go

@@ -0,0 +1,28 @@
+package llm
+
+// StreamEvent is one increment of a streaming response.
+//
+// Exactly one field group is meaningful per event: a text delta, a completed
+// tool call, or the final response. Tool-call arguments are buffered by the
+// provider until complete — consumers never see partial JSON.
+type StreamEvent struct {
+	// TextDelta is a fragment of assistant text.
+	TextDelta string
+
+	// ToolCall, when non-nil, is a fully-assembled tool call.
+	ToolCall *ToolCall
+
+	// Response, when non-nil, is the final accumulated response (content,
+	// tool calls, finish reason, usage). It is always the last event.
+	Response *Response
+}
+
+// Stream delivers a response incrementally.
+//
+// Next returns io.EOF after the final event (the one carrying Response).
+// Close releases the underlying connection and is safe to call at any time,
+// including after io.EOF or concurrently with Next returning.
+type Stream interface {
+	Next() (StreamEvent, error)
+	Close() error
+}