Nerves board: flag library config keys no library code reads (#174)

[Jammy2211]

Summary

Extends the Nerves board so it flags library config keys that no library code reads (closes #174).

scripts/board.py now walks every library package's .py files (and autonerves/) with an AST scan for config lookups: literal conf.instance[...] chains (multi-line, .get("k")), non-literal subscripts treated as wildcards, sections bound to a local and indexed later (closures included), and should_output("name"). Every library settings key is then classed used / section-read / unused against the reads of the whole stack. Priors are exempt, and the scan is skipped if any library is missing.

Where it shows up: an "unused" chip on the key, per-file counts on the repo page, a "Possibly unused config keys" section on the index (grouped by library, with GitHub links), the class in board.json and the key index, and one info item per library in state.json. It is never yellow. SOURCES gains each library's package dir, and the workflow's sparse checkout now also fetches <package>/**/*.py.

Scan results (used / section-read / unused)

Library	used	section-read	unused
PyAutoFit	58	45	8
PyAutoArray	43	84	1
PyAutoGalaxy	51	190	1
PyAutoLens	17	50	58
PyAutoCTI	26	46	8

False-positive review list (needs human review, may be read dynamically)

These PyAutoFit general.yaml keys are flagged unused. They may be read through a dynamic path the AST scan cannot see, so do not delete them on this evidence alone:

• output.log_level
• output.log_to_file
• output.log_file
• output.search_internal
• profiling.repeats

Also: PyAutoLens has a dead 57-key DynestyStatic / DynestyDynamic block in non_linear.yaml, which accounts for almost all of Lens's 58 unused keys. It is a cleanup candidate for a follow-up, not something this PR changes.

API Changes

None to the autonerves package. This is internal to the board script and workflow. The only visible schema change is additive: board.json / the key index gain a per-key usage class, and state.json gains one info item per library.
See full details below.

Test Plan

• python -m pytest test_autonerves -q: 206 passed (worktree, before push)
• nerves_board.yml renders the "Possibly unused config keys" section after merge (dispatch after merge)

Heart YELLOW acknowledged by human (reasons: manifest drift in the hub organism blurb (7), organism-map blocks (1) and workspace checkouts (1); release validation stale for PyAutoFit/Array/Galaxy/Lens). None of these involve this repo.

Full API Changes (for automation & release notes)

Added

• scripts/board.py: AST config-read scanner over library packages; per-key class used / section-read / unused
• board.json / key index: per-key usage class field
• state.json: one info item per library summarising possibly-unused keys
• .github/workflows/nerves_board.yml: sparse checkout adds <package>/**/*.py

Changed Behaviour

• Board index gains a "Possibly unused config keys" section; repo pages gain per-file counts

Closes #174

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