refactor(firmware): remove the retired simulation workflow

Remove the retired simulator workspace member, dependencies, workflow
and documentation. Keep the shared flight and hardware crates.

Validation: retirement prefix host check, command-dispatch tests and
book build.

firmware/README.md

@@ -2,16 +2,15 @@
 
 This directory is the Kaptan flight-controller firmware workspace. It owns the
 Rust workspace manifest, board declarations, platform implementation, reusable
-firmware crates, simulator, firmware scripts, and firmware test fixtures.
+firmware crates and firmware scripts.
 
 ```text
 firmware/
 ├── boards/     immutable board declarations and linker layouts
 ├── libs/       reusable no_std firmware crates
 ├── platforms/  hardware platform composition
-├── tools/      SITL and firmware development tooling
-├── scripts/    packaging, analysis, and coverage helpers
-└── tests/      firmware integration-test fixtures
+├── tools/      firmware development tooling
+└── scripts/    packaging, analysis, and coverage helpers
 ```
 
 This workspace owns its Makefile. From this directory, list its commands with: