FN-6232: centralize triage planning prompt resolution

Route standard triage prompt construction through workflow IR while keeping custom and fast-mode overrides intact.

- Export the workflow planning prompt resolver and use it as the standard triage prompt source.
- Move duplicated triage policy text into the shared core prompt definition.
- Add regression coverage for workflow-sourced planning prompts, duplicate search guidance, and prompt override behavior.
- Add a patch changeset for the published CLI bundle.

Files changed:
 .changeset/FN-6232-triage-prompt-single-source.md  |   5 +
 packages/core/src/__tests__/agent-prompts.test.ts  |  58 ++--
 packages/core/src/agent-prompts.ts                 | 111 +++++--
 packages/core/src/index.ts                         |   2 +
 packages/core/src/workflow-ir-resolver.ts          |  28 ++
 .../triage-duplicate-search-regression.test.ts     |  19 +-
 .../triage-planning-prompt-single-source.test.ts   | 179 +++++++++++
 packages/engine/src/__tests__/triage.test.ts       |  91 +++---
 packages/engine/src/triage.ts                      | 352 +--------------------
 9 files changed, 413 insertions(+), 432 deletions(-)

Fusion-Task-Id: FN-6232

Fusion-Task-Lineage: c70c33ed-c61b-49d2-bcea-d860899c1512

.changeset/FN-6232-triage-prompt-single-source.md

@@ -0,0 +1,5 @@
+---
+"@runfusion/fusion": patch
+---
+
+Resolve the standard triage planning prompt from the selected workflow IR planning node instead of the removed engine-side `TRIAGE_SYSTEM_PROMPT` duplicate. The built-in `default-triage` prompt is now the canonical policy source for `builtin:coding`; where the old copies disagreed, the surviving canonical subtask-split threshold is `MORE THAN 7 implementation steps` (with the matching `MORE THAN 3 different packages/modules` guidance). Fast-mode triage continues to use `FAST_TRIAGE_SYSTEM_PROMPT` unchanged.