feat(claude_sdk): add CSDK-019, tool prints to stdout for diagnostics

[bradAGI]

Claude SDK and MCP were the two mature packs with no observability rule. OpenAI ships OAI-010 and ADK ships ADK-009 for the same print() pattern.

Two Claude-SDK-specific consequences go beyond the lost-log-line framing OAI-010 uses:

• A Python SDK tool is commonly served to the agent over an MCP stdio transport, where stdout carries the JSON-RPC frames. A loose print interleaves with them and the client hits a parse error on a line that isn't JSON.
• When the SDK is driven programmatically, the host application is already reading the SDK's own message stream — so tool prints land interleaved with it rather than in the application's logs, attributable to no particular turn or tool call.

The fix reflects the first point: where the tool may be served over stdio, the log handler needs to write to stderr, which the transport leaves alone. Swapping print for a logger that still defaults to stdout doesn't fix that case.

Severity/confidence match OAI-010 (low / 0.65) since the dominant case here is still the lost diagnostic. The sharper stdio-corruption case is the primary reading in MCP-023 (#82), where stdio is the default transport.

Verification — engine built at main:

$ trustabl rules validate .
OK: 86 rule pack(s), 207 rule(s) valid under rule schema version 14

Fire (print(...) in a @tool-decorated handler): CSDK-002, CSDK-019, CSDK-203
Silent (logger.info(...) with basicConfig(stream=sys.stderr)): CSDK-002, CSDK-203

(CSDK-002 and CSDK-203 are pre-existing rules firing on the fixture's untyped args param and missing CLAUDE.md.)

has_print_call matches a bare print callee, so pprint and other attribute calls don't false-positive.

No new predicates, so no schema_version bump.