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/opencollection.yml

@@ -25,7 +25,9 @@ docs:
 
     Every request in this collection is GENERATED from the canonical API
     docs (docs/developer/api/**). Do not hand-edit request files — edit
-    the doc and re-run scripts/gen_bruno_from_docs.py --apply.
+    the doc, then regenerate:
+
+      go test ./internal/docs -run TestBrunoMirrorsDocs -update
 
     Hand-authored exceptions (not generated):
       - NewDevDBSetup/ — ordered dev-database bootstrap sequence