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>

crates/omnigraph-cli/src/cli.rs

@@ -29,6 +29,28 @@ pub(crate) struct Cli {
 pub(crate) enum Command {
     /// Print the CLI version
     Version,
+    /// Store a bearer token for a named server in ~/.omnigraph/credentials
+    /// (0600). Token from --token or one line on stdin:
+    /// `echo $TOKEN | omnigraph login prod`. The keyed token applies to
+    /// requests whose URL matches the server's `url` in the operator
+    /// config's `servers:` map.
+    Login {
+        /// Server name (keys the credential; declare its url under
+        /// `servers:` in ~/.omnigraph/config.yaml)
+        name: String,
+        /// The token. Prefer piping via stdin over this flag (shell
+        /// history).
+        #[arg(long)]
+        token: Option<String>,
+        #[arg(long)]
+        json: bool,
+    },
+    /// Remove a named server's stored credential. Idempotent.
+    Logout {
+        name: String,
+        #[arg(long)]
+        json: bool,
+    },
     /// Generate, clean, or refresh explicit seed embeddings
     Embed(EmbedArgs),
     /// Initialize a new graph from a schema