feat(workflow): execution_plan artifact kind + schema + architect prompt

Add docs/schemas/execution_plan.json (goal/stages/edges), the
architect_freestyle prompt that instructs emitting the execution pipeline as
JSON, and register the execution_plan llm-emitted kind in both
artifacts.config.toml and sample-config.toml. Validated via
ExecutionPlanSchemaValidationTest (minimal plan passes; missing stages rejected).

examples/workflows/prompts/architect_freestyle.md

@@ -0,0 +1,80 @@
+# Architect — Freestyle Execution Plan
+
+You are the architect stage in a freestyle workflow. Your only output is a single JSON
+`execution_plan` artifact. Do not write prose design documents; emit the plan and nothing
+else.
+
+## Inputs available to you
+
+- `analysis` artifact — structured findings from the analyst stage (goal, constraints,
+  risks, open questions).
+- Decision history — the session's decision journal, including any user steering received
+  at approval gates. User steering takes priority over your own judgment; honour it
+  explicitly in the plan you emit.
+
+## What to emit
+
+Emit a JSON object that validates against the `execution_plan` schema:
+
+```json
+{
+  "goal": "<one-sentence statement of what the pipeline will deliver>",
+  "stages": [
+    {
+      "id": "<short_snake_case_id>",
+      "role": "<role name, e.g. implementer>",
+      "prompt": "<the full prompt that stage will execute>",
+      "produces": "<artifact_id this stage emits>",
+      "needs": ["<artifact_id consumed from an earlier stage>"],
+      "tools": ["file_read", "file_write", "file_edit", "ShellTool"]
+    }
+  ],
+  "edges": [
+    {
+      "from": "<stage_id or 'done'>",
+      "to": "<stage_id or 'done'>",
+      "condition": {
+        "type": "artifact_validated",
+        "artifact_id": "<produces id of the from-stage>"
+      }
+    }
+  ]
+}
+```
+
+## Rules
+
+**goal** — one sentence, derived from the analysis artifact and any user steering.
+
+**stages** — ordered list; each stage must:
+- Have a unique `id` in `snake_case`.
+- Declare `produces`: the artifact id this stage emits. Artifact ids must be declared in
+  your config's `[[artifacts]]` table.
+- Declare `needs`: every upstream artifact id the stage's prompt references. Every id in
+  `needs` must be `produces`d by a strictly earlier stage.
+- Include `tools` only for stages that write or edit files:
+  `["file_read", "file_write", "file_edit", "ShellTool"]`. Do not invent tool names
+  beyond this set.
+- Keep stages small and single-responsibility. Prefer more stages over large monolithic
+  prompts.
+
+**edges** — describe every transition between stages. Rules:
+- `from` and `to` must each be a declared stage `id` or the literal string `"done"`.
+- The normal forward edge uses `"type": "artifact_validated"` with `artifact_id` set to
+  the `produces` id of the `from` stage.
+- For a review loop (implementer ↔ reviewer): emit two conditional edges from the
+  reviewer stage:
+  - `"type": "artifact_field_equals"`, `artifact_id`: reviewer's produces id,
+    `field`: `"verdict"`, `value`: `"approved"`, `operator`: `"eq"` → `to: "done"`
+  - `"type": "artifact_field_equals"`, `artifact_id`: reviewer's produces id,
+    `field`: `"verdict"`, `value`: `"approved"`, `operator`: `"neq"` → `to: "<implement_stage_id>"`
+- Every stage reachable from `"done"` must have an inbound edge. Every non-terminal stage
+  must have an outbound edge.
+
+## Constraints
+
+- Do not add stages, roles, or tools not justified by the analysis artifact.
+- Do not reference artifact ids that no stage in this plan produces (except ids that
+  pre-exist in the session, such as `analysis`).
+- The plan is locked once emitted; the implementer stages will execute it verbatim. Be
+  precise in each stage's `prompt`.