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>

docs/user/cli.md

@@ -42,7 +42,7 @@ omnigraph branch create --uri graph.omni --from main feature-x
 omnigraph branch list --uri graph.omni
 omnigraph branch merge --uri graph.omni feature-x --into main
 
-omnigraph ingest --data batch.jsonl --branch review/import-2026-04-09 graph.omni
+omnigraph load --data batch.jsonl --branch review/import-2026-04-09 --from main --mode merge graph.omni
 omnigraph export graph.omni --branch main --type Person > people.jsonl
 omnigraph commit list graph.omni --branch main --json
 omnigraph commit show --uri graph.omni <commit-id> --json