feat: add structured logging helpers and instrument get_issue (#14)

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.

src/aegis_gitea_mcp/logging_utils.py

@@ -20,7 +20,7 @@ class JsonLogFormatter(logging.Formatter):
 
     def format(self, record: logging.LogRecord) -> str:
         """Serialize a log record to JSON."""
-        payload = {
+        payload: dict[str, Any] = {
             "timestamp": datetime.now(timezone.utc).isoformat(),
             "level": record.levelname,
             "logger": record.name,

tests/test_logging_utils.py

@@ -7,6 +7,7 @@ import logging
 
 import pytest
 
+from aegis_gitea_mcp.config import reset_settings
 from aegis_gitea_mcp.logging_utils import (
     JsonLogFormatter,
     log_event,
@@ -18,6 +19,16 @@ from aegis_gitea_mcp.tools.read_tools import get_issue_tool
 READ_TOOLS_LOGGER = "aegis_gitea_mcp.tools.read_tools"
 
 
+@pytest.fixture(autouse=True)
+def tool_env(monkeypatch: pytest.MonkeyPatch) -> None:
+    """Provide minimal settings environment for response limit helpers."""
+    reset_settings()
+    monkeypatch.setenv("GITEA_URL", "https://gitea.example.com")
+    monkeypatch.setenv("GITEA_TOKEN", "test-token")
+    monkeypatch.setenv("MCP_API_KEYS", "a" * 64)
+    monkeypatch.setenv("ENVIRONMENT", "test")
+
+
 def test_sanitize_context_masks_sensitive_keys() -> None:
     """Sensitive keys are masked case-insensitively; others pass through."""
     cleaned = sanitize_context(
@@ -100,9 +111,7 @@ async def test_get_issue_tool_emits_debug_events(caplog: pytest.LogCaptureFixtur
     assert "get_issue.start" in events
     assert "get_issue.payload_shape" in events
 
-    field_checks = [
+    field_checks = [r.context for r in caplog.records if r.getMessage() == "get_issue.field_check"]
-        r.context for r in caplog.records if r.getMessage() == "get_issue.field_check"
-    ]
     assert {"field": "labels", "is_none": True, "value_type": None} in field_checks
     assert {"field": "user", "is_none": True, "value_type": None} in field_checks