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/tools/read_tools.py

@@ -2,6 +2,7 @@
 
 from __future__ import annotations
 
+import logging
 from typing import Any
 
 from aegis_gitea_mcp.gitea_client import (
@@ -10,6 +11,7 @@ from aegis_gitea_mcp.gitea_client import (
     GiteaClient,
     GiteaError,
 )
+from aegis_gitea_mcp.logging_utils import log_event, log_nullable_field
 from aegis_gitea_mcp.response_limits import limit_items, limit_text
 from aegis_gitea_mcp.tools.arguments import (
     CommitDiffArgs,
@@ -38,6 +40,8 @@ from aegis_gitea_mcp.tools.arguments import (
     SearchCodeArgs,
 )
 
+logger = logging.getLogger(__name__)
+
 
 async def search_code_tool(gitea: GiteaClient, arguments: dict[str, Any]) -> dict[str, Any]:
     """Search repository code and return bounded result snippets."""
@@ -265,8 +269,26 @@ async def list_issues_tool(gitea: GiteaClient, arguments: dict[str, Any]) -> dic
 async def get_issue_tool(gitea: GiteaClient, arguments: dict[str, Any]) -> dict[str, Any]:
     """Get issue details."""
     parsed = IssueArgs.model_validate(arguments)
+    log_event(
+        logger,
+        logging.DEBUG,
+        "get_issue.start",
+        owner=parsed.owner,
+        repo=parsed.repo,
+        issue_number=parsed.issue_number,
+    )
     try:
         issue = await gitea.get_issue(parsed.owner, parsed.repo, parsed.issue_number)
+        log_event(
+            logger,
+            logging.DEBUG,
+            "get_issue.payload_shape",
+            top_level_keys=sorted(issue.keys()) if issue else None,
+        )
+        # Surface nullable collections that previously broke parsing (see #13).
+        log_nullable_field(logger, "get_issue.field_check", "labels", issue.get("labels"))
+        log_nullable_field(logger, "get_issue.field_check", "assignees", issue.get("assignees"))
+        log_nullable_field(logger, "get_issue.field_check", "user", issue.get("user"))
         return {
             "number": issue.get("number", 0),
             "title": limit_text(str(issue.get("title", ""))),