Adds issue_edit, mapping to Gitea's PATCH /repos/{owner}/{repo}/issues/{index},
so an existing issue's title and/or body can be edited through the MCP surface.
Previously the only post-create mutation was issue_comment, which buries
backlinks in the thread instead of the canonical body.
Partial patch via pointer fields (omitempty): omitted fields are left
untouched, an explicit empty string clears a field. Body is sent verbatim —
no identity footer — so repeated edits are idempotent, matching the acceptance
criteria. Registered alongside the other issue tools for tool_search discovery.
Closes#34
