feat(robomp): added local issue indexing and commit-search tool support

- Added `ROBOMP_ISSUE_INDEX_SYNC_SECONDS` configuration and lifecycle-managed issue indexing through startup/shutdown hooks.
- Added issue/PR index tables with FTS5 triggers plus upsert and keyword/filter search helpers for indexed records.
- Added GitHub backend/proxy support for `IssueIndexEntry` and issue-index page retrieval, including webhook ingestion and periodic watermark-driven sync.
- Updated `gh_search_issues` to prefer local index queries when synchronized and added `search_commits` host tool with query modes and validation.

python/robomp/src/github_backend.py

@@ -13,6 +13,7 @@ from typing import Any, Protocol
 
 from robomp.github_client import (
     CommentInfo,
+    IssueIndexEntry,
     IssueInfo,
     IssueSummary,
     PullRequestFileInfo,
@@ -47,6 +48,14 @@ class GitHubBackend(Protocol):
     ) -> list[IssueSummary]: ...
 
     async def search_issues(self, repo: str, query: str, *, limit: int = 10) -> list[IssueSummary]: ...
+    async def list_issue_index_entries(
+        self,
+        repo: str,
+        *,
+        since: str | None = None,
+        page: int = 1,
+        per_page: int = 100,
+    ) -> list[IssueIndexEntry]: ...
 
     async def list_comments(self, repo: str, number: int) -> list[CommentInfo]: ...