📝 add AGENTS.md with spec-driven development workflow

Author: cowboyd

AGENTS.md

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+# AGENTS.md
+
+## Spec-driven development
+
+- Specs in `specs/` are the source of truth. Code conforms to specs,
+  not the other way around.
+
+- Never add, remove, or change a public API in code without first
+  updating the relevant spec and getting explicit approval from the
+  user. This includes changes to `Term`, `createTerm`, `render()`,
+  directive constructors (`open`, `close`, `text`), sizing helpers
+  (`grow`, `fixed`, `fit`), and any future spec'd interfaces.
+
+- The workflow is: propose the spec change, wait for approval,
+  then implement. Do not combine spec changes with implementation
+  in a single step.
+
+- The renderer and input parser are specified separately
+  (`renderer-spec.md` and `input-spec.md`). They are architecturally
+  independent. Do not introduce dependencies between them.
+
+- Each test file tests exactly one spec. Do not put tests for one
+  spec into another spec's test file.
+
+## Rendering invariants
+
+- The renderer MUST NOT perform IO. It produces bytes; the caller
+  writes them.
+
+- The renderer MUST NOT manage terminal state (alternate buffer,
+  cursor visibility, mouse reporting, keyboard protocol modes).
+
+- Each frame is a complete snapshot. The renderer carries no UI tree
+  state between frames — only cell buffers for diffing.
+
+- Directives are plain objects. No classes, no methods, no prototype
+  chains. The flat array pattern is normative.