fix(maestro): drop SHARED_ prefix from [shared] env vars

The shared section is the implicit global scope, so its fields use
the bare `MAESTRO_<FIELD>` env-var spelling. Only `[daemon]` and
`[registry]` sections get section-prefixed env vars.

Renames:
- MAESTRO_SHARED_TOKEN        → MAESTRO_TOKEN
- MAESTRO_SHARED_REGISTRY_URL → MAESTRO_REGISTRY_URL

`MAESTRO_DAEMON_*` and `MAESTRO_REGISTRY_BIND_ADDR` are unchanged.

This restores the canonical spec name (`MAESTRO_TOKEN`) that operators
already use in their deployment scripts — NAP-233 had renamed it to
`MAESTRO_SHARED_TOKEN`, which silently broke daemon auth against
existing registry deployments still setting `MAESTRO_TOKEN` in env.

Constants renamed alongside: `ENV_SHARED_TOKEN` → `ENV_TOKEN`,
`ENV_SHARED_REGISTRY_URL` → `ENV_REGISTRY_URL`. `EnvOverrides` fields
shed their `shared_` prefix to match.

Touches: daemon/cli/registry crates, e2e harness, design spec.

crates/maestro-e2e/src/daemon.rs

@@ -152,9 +152,9 @@ fn launch(
         .env("XDG_CONFIG_HOME", home.join(".config"))
         .env("XDG_STATE_HOME", home.join(".local/state"))
         .env("TMUX_TMPDIR", tmux_tmpdir)
-        .env("MAESTRO_SHARED_TOKEN", token)
+        .env("MAESTRO_TOKEN", token)
         .env("MAESTRO_DAEMON_ID", daemon_id)
-        .env("MAESTRO_SHARED_REGISTRY_URL", registry_url)
+        .env("MAESTRO_REGISTRY_URL", registry_url)
         .env("MAESTRO_DAEMON_BIND_ADDR", bind.to_string())
         .env("MAESTRO_DAEMON_URL", daemon_url)
         .env("RUST_LOG", "warn")

crates/maestro-e2e/src/registry.rs

@@ -38,7 +38,7 @@ impl Registry {
             .env("HOME", home.path())
             .env("XDG_CONFIG_HOME", home.path().join(".config"))
             .env("XDG_STATE_HOME", home.path().join(".local/state"))
-            .env("MAESTRO_SHARED_TOKEN", token)
+            .env("MAESTRO_TOKEN", token)
             .env("MAESTRO_REGISTRY_BIND_ADDR", addr.to_string())
             .env("RUST_LOG", "warn")
             .stdout(Stdio::null())