ci: restore install.rs server-symbol allowlist and worker-token scrub exemption

Both Boundary checks have been red on main for multiple commits:

- check-boundary.sh: install.rs reintroduced direct use of
  fabro_config::ServerSettings::from_layer in 93b6577cd but was dropped
  from server_symbol_allowlist in bb0d05be2. Re-add it.
- check-env-mutation.sh: the worker FABRO_WORKER_TOKEN scrub added in
  077469d0c is documented as the approved pattern in
  docs-internal/server-secrets-strategy.md but was missing from the
  allowlist. Add the exact line.

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

bin/dev/check-boundary.sh

@@ -5,6 +5,7 @@ cd "$(dirname "$0")/../.."
 
 server_symbol_allowlist=(
   "lib/crates/fabro-cli/src/local_server.rs"
+  "lib/crates/fabro-cli/src/commands/install.rs"
   "lib/crates/fabro-cli/src/commands/run/runner.rs"
   "lib/crates/fabro-cli/src/commands/pr/mod.rs"
   "lib/crates/fabro-cli/src/commands/pr/create.rs"

bin/dev/check-env-mutation.sh

@@ -21,6 +21,7 @@ while IFS= read -r match; do
   case "$path:$line" in
     "lib/crates/fabro-telemetry/src/spawn.rs:std::env::set_var(key, value);" | \
     "lib/crates/fabro-telemetry/src/spawn.rs:std::env::remove_var(key);" | \
+    'lib/crates/fabro-cli/src/main.rs:std::env::remove_var("FABRO_WORKER_TOKEN");' | \
     'lib/crates/fabro-server/src/install.rs:std::env::set_var("FABRO_TEST_IN_MEMORY_STORE", "1");')
       continue
       ;;