feat: SQL safety policy (SELECT/INSERT only) at submit time

The agent can now write PySpark that runs DROP/DELETE/UPDATE/etc. on
production tables. Add a static guard that rejects anything other than
SELECT and INSERT at two enforcement points:

  1. generate_job_file: validates BEFORE writing to disk. Agent gets
     immediate feedback ('rewrite to use only SELECT/INSERT') rather
     than learning at submit time.

  2. prepare_submit_job: re-validates the script content (reads the
     file) as a defense-in-depth check. Catches host-mounted files,
     manually-edited files, anything that bypassed generate_job_file.

How it works:
  - common/sql_guard.py extracts Python string literals whose first
    keyword is a SQL verb (catches spark.sql('...'), f-strings, and any
    raw SQL literal)
  - sqlparse splits each literal into statements; we check the first
    keyword against the policy (SELECT/INSERT/WITH allowed; DROP,
    DELETE, UPDATE, TRUNCATE, ALTER, CREATE, REPLACE, MERGE, GRANT,
    REVOKE, SET, SHOW, KILL, EXEC, etc. forbidden)
  - WITH recurses into the CTE body to catch WITH x AS (DROP ...) ...
  - The MCP layer maps ValueError -> HTTP 400 (existing handler)

Test coverage:
  - 28 unit tests in test_sql_guard.py cover: extraction (single/double/
    f-string, English false positives, multi-literal), statement
    classification (SELECT, INSERT, DROP, DELETE, UPDATE, TRUNCATE,
    ALTER, CREATE, multi-statement, CTE bodies, comments)
  - 2 integration tests verify MCP layer returns 400 with the policy
    explanation at both generate_job_file and prepare_submit_job

Limitations (documented in sql_guard.py docstring):
  - f-strings where the SQL is built at runtime (e.g. f'SELECT * FROM
    {user_input}') look like SELECTs at static-analysis time. The
    guard catches the static literal; the runtime substitution is the
    caller's responsibility.
  - pyspark.sql.functions.expr('...') accepts SQL inline; not currently
    caught. (Future work.)

146/146 still pass. Live verified: DROP TABLE -> MCP 400 with policy
explanation; SELECT -> MCP 200 + file written to ./data/jobs/.

uv.lock

@@ -849,6 +849,7 @@ dependencies = [
     { name = "loguru" },
     { name = "pydantic" },
     { name = "pydantic-core" },
+    { name = "sqlparse" },
     { name = "uvicorn" },
 ]
 
@@ -867,6 +868,7 @@ requires-dist = [
     { name = "loguru", specifier = ">=0.7.3" },
     { name = "pydantic", specifier = ">=2.13.4" },
     { name = "pydantic-core", specifier = ">=2.46.4" },
+    { name = "sqlparse", specifier = ">=0.5.0" },
     { name = "uvicorn", specifier = ">=0.49.0" },
 ]
 
@@ -876,6 +878,15 @@ dev = [
     { name = "pytest-mock", specifier = ">=3.14" },
 ]
 
+[[package]]
+name = "sqlparse"
+version = "0.5.5"
+source = { registry = "https://pypi.tuna.tsinghua.edu.cn/simple/" }
+sdist = { url = "https://pypi.tuna.tsinghua.edu.cn/packages/90/76/437d71068094df0726366574cf3432a4ed754217b436eb7429415cf2d480/sqlparse-0.5.5.tar.gz", hash = "sha256:e20d4a9b0b8585fdf63b10d30066c7c94c5d7a7ec47c889a2d83a3caa93ff28e", size = 120815, upload-time = "2025-12-19T07:17:45.073Z" }
+wheels = [
+    { url = "https://pypi.tuna.tsinghua.edu.cn/packages/49/4b/359f28a903c13438ef59ebeee215fb25da53066db67b305c125f1c6d2a25/sqlparse-0.5.5-py3-none-any.whl", hash = "sha256:12a08b3bf3eec877c519589833aed092e2444e68240a3577e8e26148acc7b1ba", size = 46138, upload-time = "2025-12-19T07:17:46.573Z" },
+]
+
 [[package]]
 name = "sse-starlette"
 version = "3.4.5"