feat(import): add stable per-resource source keys to the importer contract

Re-importing an edited document currently has no way to tell an updated
request from a new one, because every import mints fresh model IDs. Give
each imported resource a key that identifies its source element instead,
so a later import can match against what it already created.

`ImportResponse` gains an optional `sourceKeys` map, keyed by the resource
IDs plugins already emit. A parallel map keeps `ImportResources` and the
partial TS types unchanged.

Bundled importers emit keys where the format carries them: Postman item
IDs, Insomnia `_id`s, OpenAPI `operationId` (falling back to the route),
and Yaak model IDs. curl has no identity of its own, so it emits none.

Planning derives a key for anything left over, hashed from the model type,
folder ancestry, and name or method+URL, prefixed `fb:` so later code can
tell derived keys from importer-provided ones. These break when the source
document renames or moves a resource, which is accepted.

Keys land in `ImportPlan.source_keys`, keyed by minted model ID so the
mapping stays lossless, ready for the commit step to persist. Nothing
consumes them yet.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

plugins/importer-postman/tests/fixtures/params.output.json

@@ -100,5 +100,8 @@
       }
     ],
     "folders": []
+  },
+  "sourceKeys": {
+    "GENERATE_ID::WORKSPACE_2": "collection:9e6dfada-256c-49ea-a38f-7d1b05b7ca2d"
   }
 }