cowork-bot: automated improvements (cowork/improve-json2sql-2) (#36)

* cowork-bot: emit valid SQL for mixed-type columns and empty/nested-only roots

Type inference now collapses a column whose values mix incompatible types
(e.g. a string and an integer) to TEXT instead of keeping the first-seen
numeric type. Previously this produced INSERT statements with a quoted string
literal in a numeric column, which Postgres/MySQL reject as invalid SQL. A
NULL value is treated as "unset" so a column that first sees NULL can still
take a concrete type when a non-NULL value arrives (e.g. [null, 42] ->
INTEGER), and an all-NULL column resolves to TEXT.

convert()/generate_schema() no longer emit an invalid `CREATE TABLE "x" ();`
or `INSERT INTO "x" () VALUES ();` for an empty object or a root whose only
content is nested arrays (flatten). They now emit only the valid child
tables, or an explicit comment when nothing can be generated, instead of a
silent green no-op.

- src/json2sql/converter.py: add _infer_type/_merge_type; skip empty root tables
- tests/test_edge_cases.py: correct the two tests that encoded the old buggy
  widening behavior
- tests/test_type_inference.py: regression tests for mixed-type, NULL, and
  empty/nested-only-root cases across all three dialects
- .github/workflows/cowork-auto-pr.yml: seed server-side PR opener

* fix: remove false PyPI badge and broken json2sql-cli install commands

* chore: gitignore scratch file _run_tests.py (cron artifact, not project code)

* chore: gitignore additional scratch files (_audit_reqs.txt, _pr_body.md)

Author: Coding-Dev-Tools

.gitignore

@@ -73,5 +73,10 @@ research/
 fixtures/generated/
 .ruff_cache/
 
+# Scratch/debug files (cron artifacts)
+_run_tests.py
+_audit_reqs.txt
+_pr_body.md
+
 # Added by release-prep
 node_modules