S24.06: Retire Example/FreeRtos/HelloWorld bring-up smoke binary

[DavidCozens]

Parent epic: #254 (E24: Code Hygiene)

Should land before: #333 (S24.05 moves Example/FreeRtos/SingleTask/ → Bdd/Targets/FreeRtos/. Retiring HelloWorld first lets that story relocate a clean single-child tree instead of a two-child one that needs splitting mid-move.)

Overview

Example/FreeRtos/HelloWorld/ is a minimal QEMU bring-up smoke ELF
— one task printing one line over the CMSDK UART, no IP stack, no
syslog. It was the right shape during S08 bring-up when the
cross-compile / scheduler / UART / newlib chain was the unknown.
After S08.03 + S08.04 closed out, Example/FreeRtos/SingleTask/
exercises every layer HelloWorld did, plus FreeRTOS-Plus-TCP, plus
the full library, plus the BDD scenarios. The diagnostic separation
HelloWorld provided is no longer load-bearing.

Why

• SingleTask is a strict superset of HelloWorld at runtime. If
  SingleTask hangs before the SolidSyslog> prompt, the
  toolchain/scheduler/UART/newlib chain is suspect — same signal
  HelloWorld gave. After the prompt, IP/syslog is in play.
• The HelloWorld retirement was already flagged as a slice-3 gate
  in the S08.04 plan (see DEVLOG 2026-05-11 "S08.04 slice 2"
  Deferred to slice 3 — HelloWorld retirement decision).
• Pre-launch (v0.0.0); no external readers.
• Carrying both means two QEMU smoke steps in CI's
  build-freertos-target job and two VS Code launch/task entries
  to keep in sync.

In scope

• Delete Example/FreeRtos/HelloWorld/ (directory + its
  add_subdirectory(HelloWorld) line in
  Example/FreeRtos/CMakeLists.txt).
• .github/workflows/ci.yml — build-freertos-target job:
  • Drop SolidSyslogFreeRtosHelloWorld from the cmake --build --target ... list.
  • Drop the HelloWorld QEMU smoke step that runs the ELF and
    asserts on its stdout.
  • The SingleTask cross-build remains; it is itself the smoke.
• .vscode/tasks.json — the freertos-cross branch of the
  build-and-test task currently builds
  SolidSyslogFreeRtosHelloWorld. Switch to
  SolidSyslogFreeRtosSingleTask. The "run on QEMU (FreeRTOS)"
  task's -kernel ...HelloWorld.elf path becomes
  ...SingleTask/SolidSyslogFreeRtosSingleTask.elf.
• .vscode/launch.json — rename / repoint the
  Debug FreeRTOS HelloWorld (QEMU) launch config to the
  SingleTask ELF. (Bikeshed: "Debug FreeRTOS SingleTask (QEMU)".)
• Example/FreeRtos/README.md — rewrite the table (one row,
  SingleTask) and update the VS Code Ctrl+Shift+B / Run-task /
  command-line / GDB sections to reference the SingleTask ELF
  path.
• docs/ci.md — update the build-freertos-target row (the
  "QEMU bring-up smoke for HelloWorld" wording).
• docs/containers.md — update the Debug FreeRTOS HelloWorld (QEMU) reference to match the renamed launch config.
• Example/FreeRtos/Common/CMSDK_UART.md — strip the "HelloWorld"
  contextual references (the doc is still useful for SingleTask).
• Example/FreeRtos/cmake/arm-none-eabi.cmake — comment-only
  cleanup of the HelloWorld / SingleTask mention.

Acceptance criteria

• Example/FreeRtos/HelloWorld/ no longer exists.
• build-freertos-target CI job is green, builds only the
  SingleTask ELF, and still serves the artefact to
  bdd-freertos-qemu.
• bdd-freertos-qemu is green.
• No references to SolidSyslogFreeRtosHelloWorld or
  HelloWorld.elf remain in CMakeLists, CI workflow, VS Code
  config, or live docs (DEVLOG history is exempt).
• VS Code build/run/debug tasks under the freertos-target
  service drive SingleTask end-to-end.

Out of scope

• The wider Example/FreeRtos/SingleTask/ → Bdd/Targets/FreeRtos/
  move — that's S24.05 S24.05: Move BDD-target binaries from Example/ to Bdd/Targets/<platform>/ #333.
• Any change to the BDD harness, the SingleTask main, or the
  Common/ shared infrastructure.

Risks

• After deletion, the only thing that exercises the cross-compile
  • QEMU boot path under CI is SingleTask. If SingleTask fails to
    build before reaching main, CI will surface the same failure
    HelloWorld would have — but the diff is one toolchain bring-up
    vs. a binary that also pulls in FreeRTOS-Plus-TCP and the
    library. If the toolchain regressions we've actually hit in CI
    history were FreeRTOS-Plus-TCP-aware, this is a net-zero. If
    any were lower-layer (linker script, vector table, newlib
    retargeting), SingleTask still catches them — but with a longer
    "where in the boot did it fail" investigation window.
• Mitigation: SingleTask's main boots a Service task before
  the IP stack is up, and the BDD harness times out on a missing
  prompt — same human-readable signal HelloWorld's stdout gave.

References

• Memory: project_freertos_helloworld_retirement (watching
  brief carried through S08.04).
• DEVLOG 2026-05-11 "S08.04 slice 2: FreeRTOS example switches
  to CircularBuffer + FreeRtosMutex" — Deferred to slice 3
  flagged the retirement gate.
• E24 E24: Code Hygiene #254 charter — "Other code-quality work that doesn't
  belong under a capability-focused epic".