feat(cli)!: unified load command; deprecate ingest as an alias

omnigraph load is now the single data-write command:
- works against remote graphs (POSTs the server's /ingest endpoint with the
  same bearer/actor resolution as other remote commands) — previously load
  was the only data command forced to open Lance storage directly
- --from <base> opts into fork-if-missing for --branch (the former ingest
  semantics); without --from a missing branch is an error, never a fork
- --mode is now required: overwrite is destructive, so there is no implicit
  default (the old silent default was overwrite)
- output gains base_branch/branch_created (and table sums on remote loads)

omnigraph ingest stays as a deprecated alias (defaults preserved: --from
main --mode merge) that prints a one-line warning to stderr, matching the
read/change deprecation convention; removal in a later release.

Docs updated in the same change: cli.md, cli-reference.md, policy.md,
audit.md, execution.md (unified load section), AGENTS.md quick-flow,
README.md.

BREAKING CHANGE: scripts running omnigraph load without --mode must now
pass it explicitly (previously defaulted to the destructive overwrite).

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

crates/omnigraph-cli/tests/support/mod.rs

@@ -93,7 +93,15 @@ pub fn init_graph(graph: &Path) {
 
 pub fn load_fixture(graph: &Path) {
     let data = fixture("test.jsonl");
-    output_success(cli().arg("load").arg("--data").arg(&data).arg(graph));
+    output_success(
+        cli()
+            .arg("load")
+            .arg("--mode")
+            .arg("overwrite")
+            .arg("--data")
+            .arg(&data)
+            .arg(graph),
+    );
 }
 
 pub fn write_jsonl(path: &Path, rows: &str) {