refactor(cli): collapse export + graphs-list onto GraphClient (RFC-009 Phase 3c) (#213)

The last two embedded-vs-remote forks move onto the enum, so every such
`if` in the CLI now lives in client.rs — the point of the refactor.

- `export<W: Write>`: the streaming verb 3b deferred (writes to a writer,
  chunks the HTTP response body, rather than returning a DTO). Embedded
  calls db.export_jsonl_to_writer; Remote streams the chunked body through.
  Opens WITHOUT policy (like reads), so it routes via resolve().
- `list_graphs`: remote-only by design (no local enumeration endpoint), so
  the Embedded arm keeps the loud "requires a remote multi-graph server"
  bail verbatim. Routing it through the enum still buys the shared
  resolve() addressing/token preamble the arm hand-rolled.

Retire the now-orphaned execute_export_to_writer /
execute_export_remote_to_writer pair, and sweep two pre-existing dead fns
while in the files: inferred_config_path (helpers.rs) and yaml_string
(output.rs, shadowed by test-local copies).

parity_matrix gains one row, parity_export — the single intended matrix
change in this phase. Export is a JSONL stream, not a single --json doc,
so it compares the two arms' output line-wise (sorted; twin graphs are
byte-copies so rows need no scrubbing). graphs-list gets no row: its
remote-only behavior is a documented exclusion, not an equality case.

Full workspace tests pass; all 12 parity rows green.

Co-authored-by: Claude Opus 4.8 <noreply@anthropic.com>

crates/omnigraph-cli/src/output.rs

@@ -812,10 +812,6 @@ pub(crate) fn print_policy_explain(decision: &PolicyDecision, actor_id: &str, re
     println!("message: {}", decision.message);
 }
 
-pub(crate) fn yaml_string(value: &str) -> String {
-    format!("'{}'", value.replace('\'', "''"))
-}
-
 #[derive(serde::Serialize)]
 pub(crate) struct QueriesIssue {
     pub(crate) query: String,