feat(cli): keyed credentials — servers:, the token chain, login/logout (RFC-007 PR 2)

The operator config gains servers: (name -> url; never a token). A remote
command whose URL prefix-matches an operator server resolves its bearer
token through the keyed chain first — OMNIGRAPH_TOKEN_<NAME> env, then the
[<name>] section of ~/.omnigraph/credentials (created 0600 via temp+rename,
#139 finding 7; group/world-readable files refused loudly) — falling
through to the legacy chain unchanged. URL keying makes §D5 rule 3
structural: a token is only ever sent to the server it is keyed to.
Longest-prefix matching with a path-boundary check (http://h:8080 never
matches http://h:8080-evil). Inserting the keyed hop above the legacy chain
is safe by construction — no existing setup can have servers: defined.

omnigraph login <name> stores/rotates one section (token from --token or
one stdin line — the pipe flow keeps secrets out of shell history);
omnigraph logout removes it, idempotently; logging in before declaring the
server warns instead of failing (the gh model).

Coverage: URL-match/no-substring-trap, credentials round-trip preserving
sibling sections, 0600 write + over-permissive refusal, env-name mapping;
the legacy resolve test is now hermetic against a real ~/.omnigraph and
asserts byte-identical legacy behavior with no servers defined; one
spawned-binary e2e walks the whole lifecycle against an authed server:
refusal -> wrong-token login (stdin) -> rotate (--token) -> authorized read
-> env-beats-file -> non-matching-URL negative -> logout revokes.

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

docs/dev/rfc-007-operator-config.md

@@ -241,13 +241,13 @@ future checkout-supplied surface:
 
 Three PRs, each independently useful, each landable without the next:
 
-1. **PR 1 — the operator file + identity.** Loader for
+1. **PR 1 — the operator file + identity** *(landed: #196)*. Loader for
    `~/.omnigraph/config.yaml` (+ `OMNIGRAPH_HOME`, `~`-expansion, warn-only
    unknown keys), `operator.actor` joining the `--as` cascade,
    `defaults.output` joining the format cascade, `OMNIGRAPH_CONFIG` env for
    the CLI's `--config`. Docs: `cli-reference.md` gains the two-surface
    table.
-2. **PR 2 — keyed credentials.** `servers:` in the operator layer, the
+2. **PR 2 — keyed credentials** *(landed)*. `servers:` in the operator layer, the
    §D4 chain (env + credentials file), the §D5 trust rules, and
    `omnigraph login <name>` (atomic write, `0600`). Legacy mechanisms
    untouched and tested-as-untouched.