fix(shannon-worker): insert each Shannon helper independently

[peteromallet]

Bug

The Shannon entrypoint patcher in _ensure_shannon_parent_timeout_control
defined three TypeScript helpers (isRootProcess, rootSafeClaudeArgs,
maybeSendStartupEnterKeys) as one multi-line blob and gated insertion of
the entire blob on whether rootSafeClaudeArgs was already present in the
file.

That gate fails on a partially-patched entrypoint. If an earlier megaplan
revision had injected only the first two helpers (no
maybeSendStartupEnterKeys), a newer megaplan would see
rootSafeClaudeArgs already present and skip the entire blob -- leaving
maybeSendStartupEnterKeys undefined. The downstream patch that inserts
void maybeSendStartupEnterKeys(tmuxSession); runs independently and
still spliced the call site, so the patched entrypoint compiled but
exploded at runtime with a Bun ReferenceError.

Megaplan surfaced this as Shannon readiness probe failed with exit code 1.

Fix

Split the helper blob into three independent strings and gate each on its
own unique signature substring. Insertion order is preserved by inserting
in reverse before the same buildClaudeArgs anchor, so a fully
unpatched entrypoint still ends up with isRootProcess ->
rootSafeClaudeArgs -> maybeSendStartupEnterKeys ->
buildClaudeArgs, while a partially-patched entrypoint only gets the
missing helper(s) inserted.

Test plan

• pytest tests/test_workers.py -k shannon -- existing patcher test
  still passes (full unpatched entrypoint).
• New test_shannon_worker_heals_partially_patched_entrypoint
  passes -- starts from an entrypoint that already contains
  isRootProcess and rootSafeClaudeArgs (and the dangling
  void maybeSendStartupEnterKeys(tmuxSession); call site) and asserts
  the patch pass now inserts the missing definition without duplicating
  the present helpers.
• Pre-existing test_run_shannon_step_passes_prompt_with_print_flag
  failure is unrelated to this change (fails on main too).

Scope note

The Shannon helper-patcher infrastructure plus a related non-root Shannon
runtime feature (MEGAPLAN_SHANNON_DROP_ROOT / nonroot home seeding)
and a defensive _normalize_worker_payload defaulting tweak were
already staged in the working tree but not yet committed on main -- so
this PR also carries those in-flight files. The gate-fix patch sits on
top of that infrastructure; cherry-picking only the gate refactor would
modify code that doesn't yet exist on main.