feat: add issue_edit tool (#34)

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

cmd/gitea-mcp/main.go

@@ -59,6 +59,7 @@ func main() {
 	reg.Register(tools.NewWorkflowRunStatus(giteaClient, ownerAllow))
 	reg.Register(tools.NewCodeSearch(giteaClient, ownerAllow))
 	reg.Register(tools.NewIssueCreate(giteaClient, ownerAllow))
+	reg.Register(tools.NewIssueEdit(giteaClient, ownerAllow))
 	reg.Register(tools.NewIssueComment(giteaClient, ownerAllow))
 	reg.Register(tools.NewCreateProjectFromTemplate(giteaClient, ownerAllow, "mathias", "template-go-web"))
 	reg.Register(tools.NewTagCreate(giteaClient, ownerAllow))