bruno: replace Python generator with Go golden-writer in the drift test

The generator moves into the Go toolchain per repo preference:
internal/docs/bruno_gen.go emits the OpenCollection mirrors (same output
contract — gate-verified example-request bodies, env-var params,
device-token headers, login/register/refresh token-capture scripts) and
TestBrunoMirrorsDocs grows a -update flag: verification by default,
regeneration on demand:

  go test ./internal/docs -run TestBrunoMirrorsDocs -update

Generator and checker are now the same code — they cannot drift apart.
scripts/gen_bruno_from_docs.py deleted; opencollection.yml README points
at the go-test command.

Also fixed vs the Python pass: mirror placement follows the docs tree
folder (RelPath), not the area frontmatter (several docs — highlights,
notes, ratings, users, progress — carry a different area); docs blocks
emit the collection's mapping form (content + type), not a bare scalar;
JSON body sanity is checked at generation time.

Idempotent: back-to-back -update runs are byte-identical.
make test green (9 pkgs).

bruno/collections/add_auto_assign_rule.yml

@@ -23,9 +23,11 @@ settings:
   followRedirects: true
   maxRedirects: 5
 
-docs: |-
-  # Add Auto-Assign Rule
+docs:
+  content: |-
+    # Add Auto-Assign Rule
 
-  `POST /api/collections/:id/rules` — auth: jwt.
+    `POST /api/collections/:id/rules` — auth: jwt.
 
-  Canonical reference: `docs/developer/api/collections/add_auto_assign_rule.md` (docs site → Developer → API). This request is GENERATED from that doc; edit the doc, not this file.
+    Canonical reference: docs/developer/api/collections/add_auto_assign_rule.md (docs site → Developer → API). This request is GENERATED from that doc; edit the doc, then run the drift test with -update.
+  type: text/markdown