feat(repo_update): tool for archiving + metadata patches

Adds a repo_update tool exposing PATCH /api/v1/repos/{owner}/{name}
with optional pointer fields (archived, description, private,
website, template). Only fields set by the caller are sent on the
wire, so the server patches exactly what was asked for.

Originally needed to archive ingestion-svc cleanly instead of
leaving a README tombstone, and to flip template-go-{agent,web}
to template=true so create_project_from_template stops failing
the "is not marked as template" guard.

Wire-level enforcement of "at least one field" returns ErrValidation
before any network call, preventing no-op PATCHes.

private=false (making a repo public) is allowed but flagged in the
tool description with a "verify intent before calling" warning.
The earlier issue draft suggested an ntfy confirmation hook for
that path — out of scope for this PR; the warning string is the
minimum that fits inside the tool surface today.

Wires NewRepoUpdate into cmd/gitea-mcp/main.go alongside the rest
of the repo_* family.

Closes #12

internal/gitea/repos.go

@@ -216,6 +216,8 @@ type UpdateRepoArgs struct {
 	Private       *bool   `json:"private,omitempty"`
 	Website       *string `json:"website,omitempty"`
 	DefaultBranch *string `json:"default_branch,omitempty"`
+	Archived      *bool   `json:"archived,omitempty"`
+	Template      *bool   `json:"template,omitempty"`
 }
 
 func (c *Client) UpdateRepo(ctx context.Context, owner, name string, args UpdateRepoArgs) (*Repo, error) {
@@ -253,3 +255,4 @@ func (c *Client) GetRepo(ctx context.Context, owner, name string) (*Repo, error)
 	}
 	return &r, nil
 }
+