refactor: remove howtogalaxy tutorial dirs (now in PyAutoLabs/HowToGalaxy)

[Jammy2211]

Overview

Sub-task 2 of 3 of the HowToGalaxy extraction (parent tracker was #35, auto-closed by the bootstrap PR).

Sub-task 1 shipped PyAutoLabs/HowToGalaxy#1 at tag 2026.4.13.6, copying the tutorial series from this workspace into its own repo. This issue covers removing scripts/howtogalaxy/ and notebooks/howtogalaxy/ from this workspace and fixing the cross-references.

Plan

• Move scripts/howtogalaxy/simulators/sersic.py → scripts/imaging/simulator_sersic.py (sits alongside existing simulator.py and simulator_sample.py). This simulator produces dataset/imaging/simple__sersic and is used by 9 non-howtogalaxy scripts outside the tutorial tree.
• Delete scripts/howtogalaxy/ and notebooks/howtogalaxy/.
• Rewrite the subprocess path scripts/howtogalaxy/simulators/sersic.py → scripts/imaging/simulator_sersic.py in 9 scripts + 9 matching notebooks.
• Rewrite prose pointers in README.rst, start_here.py, start_here.ipynb to point at https://github.com/PyAutoLabs/HowToGalaxy instead of notebooks/howtogalaxy.
• Smoke-test one shapelets script + one guides script under PYAUTO_TEST_MODE=2 to confirm the new subprocess path works.

Detailed implementation plan

Affected callers (subprocess path rewrite)

Scripts:

• `scripts/imaging/features/shapelets/{fit,modeling}.py`
• `scripts/imaging/features/pixelization/{fit,modeling,source_science}.py`
• `scripts/guides/modeling/customize.py`
• `scripts/guides/plot/examples/searches.py`
• `scripts/guides/advanced/over_sampling.py`
• `scripts/multi/plot.py`

Notebooks (matching set, JSON-escaped subprocess string):

• `notebooks/imaging/features/shapelets/{fit,modeling}.ipynb`
• `notebooks/imaging/features/pixelization/{fit,modeling,source_science}.ipynb`
• `notebooks/guides/modeling/customize.ipynb`
• `notebooks/guides/plot/examples/searches.ipynb`
• `notebooks/guides/advanced/over_sampling.ipynb`
• `notebooks/multi/plot.ipynb`

Prose rewrites

• `README.rst` — any "explore notebooks/howtogalaxy" → external repo link
• `start_here.py` / `start_here.ipynb` — same

Out of scope for this PR

• PyAutoGalaxy docs / Colab URLs (sub-task 3 of 3, separate issue)
• PyAutoBuild `howtogalaxy` target registration (separate follow-up)
• Content-alignment pass on HowToGalaxy chapter-1 tutorials 0 and 3 (separate follow-up on the new repo)

Branch: `feature/howtogalaxy-sub2`
Worktree root: `~/Code/PyAutoLabs-wt/howtogalaxy-sub2/`
No version bump — structural refactor, not functional.

Follows the HowToLens extraction pattern, but improves on it by actually wiring the non-tutorial callers to a workspace-level simulator rather than leaving broken subprocess paths.