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/content.go

@@ -0,0 +1,39 @@
+package llm
+
+// Part is one piece of message content: text, an image, or future media
+// kinds. The set of implementations is closed (sealed by the unexported
+// method) so providers can switch exhaustively over content kinds.
+//
+// Why: providers need a finite, known content vocabulary to serialize into
+// their wire formats; an open interface would silently drop unknown content.
+type Part interface {
+	isPart()
+}
+
+// TextPart is plain text content.
+type TextPart struct {
+	Text string
+}
+
+func (TextPart) isPart() {}
+
+// ImagePart is image content carried as raw bytes plus a MIME type.
+//
+// Why bytes-only (no URL form): the media pipeline must be able to inspect,
+// downscale, and re-encode every image to fit the target's capabilities, and
+// that requires the bytes. Callers with a URL fetch it themselves; majordomo
+// does not download remote content on a caller's behalf.
+type ImagePart struct {
+	// MIME is the image content type, e.g. "image/png" or "image/jpeg".
+	MIME string
+	// Data is the raw, unencoded image bytes (providers base64 as needed).
+	Data []byte
+}
+
+func (ImagePart) isPart() {}
+
+// Text constructs a text content part.
+func Text(s string) Part { return TextPart{Text: s} }
+
+// Image constructs an image content part from raw bytes.
+func Image(mime string, data []byte) Part { return ImagePart{MIME: mime, Data: data} }