feat(agentfabric): Add support for artifact and status update echo node types

[Drake-Ash]

What

Redesigns the AgentFabric echo node around A2A v1 task-update events, replacing the single
a2a:response variant with two purpose-built event types, and adds graph-aware lint rules to
enforce correct terminal behavior.

The echo node now supports two kind variants:

• a2a:status_update_event — sets the task state (required; an A2A v1 TASK_STATE_*
  value) with an optional message.
• a2a:artifact_update_event — emits an artifact (required) with optional append and
  lastChunk flags.

echo setStatus:
  kind: "a2a:status_update_event"
  state: "TASK_STATE_COMPLETED"
  message: a2a.message({
    messageId: uuid(),
    parts: [ a2a.textPart("You have been onboarded! ...") ]
  })

echo addArtifact:
  kind: "a2a:artifact_update_event"
  artifact: a2a.artifact({ artifactId: uuid(), name: "results", parts: [ a2a.textPart("Analysis
complete") ] })
  append: False
  lastChunk: False

Why

The previous a2a:response echo wrapped a full A2A Task object (via a2a.task(...)) and only
supported non-streaming responses. Modeling responses as discrete A2A v1 task-update events
(status vs. artifact) aligns the dialect with the A2A v1 task lifecycle, enables
incremental/streaming artifact updates, and lets the linter statically guarantee that every
execution path drives the task to a terminal state.

How

Schema (src/schema.ts)

• Replaced the a2a:response variant (with task/artifacts/message/metadata) with the two
  event variants above.
• a2a:status_update_event: state is required and constrained to the A2A v1 TASK_STATE_*
  enum; message is optional.
• a2a:artifact_update_event: artifact is required; append/lastChunk are optional
  booleans.
• Exported A2A_TASK_STATES (the 8 TASK_STATE_* values) and A2A_TERMINAL_STATES for shared
  use across schema, compiler, and linter.
• Removed the a2a.task namespaced function; a2a.message / textPart / dataPart /
  filePart / artifact / parts remain.

Lint rules

• terminal-requires-status-update (new, terminal-status-rules.ts): every terminal branch
  in the graph must reach an a2a:status_update_event echo with a terminal state. Uses graph
  extraction + reverse-BFS so the status echo need not be the leaf node (a branch may continue
  afterward, e.g. to emit an artifact or run cleanup).
• echo-invalid-state (new, in echo-rules.ts): validates that a status-update echo's
  state is a known A2A v1 task state.
• Removed echo-task-or-message-required, which no longer applies under the event model.

Other

• Bumped @agentscript/agentfabric-dialect 0.1.30 → 0.4.0 (breaking) and added a CHANGELOG
  entry.
• Updated the AgentFabric docs (apps/docs) for the new echo node, field tables, and a2a
  namespace notes.
• Migrated test fixtures (.agent / .yaml / .graph.json) and all inline test sources to the
  new event variants; the customer-support fixture now also exercises a chained status_update →
  artifact_update echo.

Breaking change / migration

Flows using kind: "a2a:response", the task: or artifacts: echo fields, or a2a.task(...)
must migrate. Replace the response with a status-update echo (set a terminal TASK_STATE_*) and,
where artifacts were attached, an a2a:artifact_update_event echo:

-echo done:
-  kind: "a2a:response"
-  task: a2a.task({ state: "completed", message: a2a.message({ ... }) })
+echo done:
+  kind: "a2a:status_update_event"
+  state: "TASK_STATE_COMPLETED"
+  message: a2a.message({ ... })

Test Plan

• Existing tests pass — pnpm --filter @agentscript/agentfabric-dialect test → 144
  passed (incl. 61 lint tests)
• New/updated tests cover the change — added cases for terminal-requires-status-update
  (terminal node not an echo, non-terminal state, valid terminal echo,
  status-echo-then-more-nodes), echo-invalid-state, and unknown-variant for the removed
  a2a:response kind
• Type checks pass — pnpm --filter @agentscript/agentfabric-dialect typecheck clean

Checklist

• My code follows the project's coding style
• I have reviewed my own diff
• I have added/updated documentation as needed (apps/docs + dialect CHANGELOG)
• This change does not introduce new warnings