mr-668: close init re-init footgun via InitOptions preflight (green)

`Omnigraph::init` is "create a new graph"; existing graphs need
an explicit overwrite. Today's behavior — silently overwrite
schema files, then on inner failure delete them via best-effort
cleanup — is destructive against an existing graph regardless of
which branch fires.

Correct-by-design fix:

* New `InitOptions { force: bool }` struct (default `force: false`).
* New `Omnigraph::init_with_options(uri, schema, options)`. The
  old `Omnigraph::init(uri, schema)` is a thin shortcut that
  passes `InitOptions::default()`.
* `init_with_storage` runs a `storage.exists()` preflight on the
  three schema URIs BEFORE any parse, write, or coordinator call.
  Any hit → typed `OmniError::AlreadyInitialized { uri }`. The
  destructive code paths (the `write_text` overwrite and the
  best-effort cleanup) are now unreachable in strict mode against
  an existing graph.
* `force: true` skips the preflight; existing operators who
  actually mean to overwrite opt in explicitly.
* CLI: `omnigraph init --force` maps to `InitOptions { force: true }`.
* HTTP: `OmniError::AlreadyInitialized` maps to 409 via
  `ApiError::from_omni`. Not currently HTTP-reachable (POST /graphs
  was pulled), but the wiring lands here so a future runtime
  create endpoint has one canonical translation.

Closes the "init is destructive against existing state" class.
The regression test added in the previous commit
(`init_on_existing_graph_uri_does_not_destroy_existing_schema`)
turns green: the original schema files now survive a second
init attempt byte-for-byte, and the call errors cleanly with
`AlreadyInitialized`. The four existing
`init_failpoint_after_*_cleans_up_*` tests stay green — strict
mode's preflight passes on a fresh tempdir, and cleanup still
runs as before when a failpoint fires mid-write.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

crates/omnigraph-cli/src/main.rs

@@ -73,6 +73,13 @@ enum Command {
         schema: PathBuf,
         /// Graph URI (local path or s3://)
         uri: String,
+        /// Overwrite existing schema artifacts at the URI. Without
+        /// this flag, init refuses to touch a URI that already holds
+        /// `_schema.pg`, `_schema.ir.json`, or `__schema_state.json`
+        /// — closes the re-init footgun (MR-668 follow-up). With the
+        /// flag, the operator opts in to destructive semantics.
+        #[arg(long)]
+        force: bool,
     },
     /// Load data into a graph
     Load {
@@ -1746,10 +1753,15 @@ async fn main() -> Result<()> {
                 print_embed_human(&output);
             }
         }
-        Command::Init { schema, uri } => {
+        Command::Init { schema, uri, force } => {
             let schema_source = fs::read_to_string(&schema)?;
             ensure_local_graph_parent(&uri)?;
-            Omnigraph::init(&uri, &schema_source).await?;
+            Omnigraph::init_with_options(
+                &uri,
+                &schema_source,
+                omnigraph::db::InitOptions { force },
+            )
+            .await?;
             scaffold_config_if_missing(&uri)?;
             println!("initialized {}", uri);
         }