refactor(cli,server): rename the --target flag to --graph (no alias)

The graph-selection flag is now `--graph` (the canonical noun, matching
`graphs:`/`cli.graph`) with NO `--target` compat alias — a clean break, to avoid
two-names-for-one-thing confusion. Applied across the 21 `omnigraph` subcommands
and the `omnigraph-server` binary (the field stays `target` internally;
`value_name = "GRAPH"` keeps help text clean). Updates the user-facing "no graph to
serve" and stored-query selection-hint error messages, plus the two cli.rs tests
that drove `--target`.

Breaking: `--target` is removed (now an unknown-flag error); use `--graph`. `uri`
is unchanged (positional or `--uri` per command — deliberately not folded into a
shared arg, since the two uri patterns can't share one flattened definition without
a breaking uri-form change). `--target-branch` (policy explain) is unrelated and
untouched. Code comments and docs still referencing `--target` are swept in L8 with
the docs.

crates/omnigraph-server/src/lib.rs

@@ -1050,8 +1050,8 @@ pub fn load_server_settings(
     } else {
         // Rule 5 → error with migration hint.
         bail!(
-            "no graph to serve: pass a URI (`omnigraph-server <URI>`), select a target \
-             (`--target <name> --config omnigraph.yaml`), set `server.graph: <name>` in \
+            "no graph to serve: pass a URI (`omnigraph-server <URI>`), select a graph \
+             (`--graph <name> --config omnigraph.yaml`), set `server.graph: <name>` in \
              omnigraph.yaml, or for multi-graph mode add a `graphs:` map to the config \
              file referenced by `--config`."
         );

crates/omnigraph-server/src/main.rs

@@ -10,7 +10,7 @@ use omnigraph_server::{ServerConfig, init_tracing, load_server_settings, serve};
 struct Cli {
     /// Graph URI
     uri: Option<String>,
-    #[arg(long)]
+    #[arg(long = "graph", value_name = "GRAPH")]
     target: Option<String>,
     #[arg(long)]
     config: Option<PathBuf>,