Polish schema endpoint: rename show, align field name, add tests

Review feedback on #23, applied on top of the original commit:

- Rename the CLI subcommand from `schema get` to `schema show` to match
  the existing `run show` / `commit show` convention. A `#[command(alias
  = "get")]` preserves muscle memory for anyone who already typed `get`.
- Rename `SchemaGetOutput` → `SchemaOutput` and its field `source` →
  `schema_source`, so the get response and the apply request use the
  same field name for the same concept.
- Use `println!` instead of `print!` in the CLI so the shell prompt
  doesn't land on the last line of schema output.
- Add three integration tests on `/schema`: happy path (no auth),
  401 when bearer is required but missing, 403 when the policy grants
  the actor branch_create but not read.

Follow-ups left for a separate PR: include `schema_ir_hash` and
`schema_identity_version` in the response payload so clients can do
drift detection and the server can set an ETag; and a fast-path local
read that skips `Omnigraph::open()` when only the schema source is
needed.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

crates/omnigraph-server/src/api.rs

@@ -281,8 +281,8 @@ pub struct SchemaApplyOutput {
 }
 
 #[derive(Debug, Clone, Serialize, Deserialize, ToSchema)]
-pub struct SchemaGetOutput {
-    pub source: String,
+pub struct SchemaOutput {
+    pub schema_source: String,
 }
 
 #[derive(Debug, Clone, Serialize, Deserialize, ToSchema)]