Adds reusable, secret-safe logging helpers to `logging_utils`:
- `log_event(logger, level, event, **context)` emits a named event with a
sanitized `context` mapping (sensitive keys masked as `***`).
- `log_nullable_field(...)` records whether a parsed field is None plus its
runtime type, without dumping its contents.
- `sanitize_context(...)` is the shared masking primitive.
The JSON formatter now serializes a record's `context` into the payload.
`get_issue_tool` is instrumented at DEBUG (`get_issue.start`,
`get_issue.payload_shape`, `get_issue.field_check` for labels/assignees/user)
so the nullable-field parsing that caused #13 is diagnosable going forward.
Adds tests for the helpers, the formatter, and the get_issue instrumentation,
and documents the pattern in docs/observability.md.
Latte