Explorer uncertainty and strict lint

[SebastianAment]

Explorer uncertainty consistency + strict-lint hardening

Two cleanups following the V2 strength GP open-source PR.

1. Consistent ±2σ uncertainty in the explorer (f974a55)

The strength-curve plot has always shaded mean ± 2σ, but the cost readout displayed ± 1σ — same ± symbol, two different conventions in the same view.

• docs/ui.mjs::updateUI — cost readout now reads ± {2σ value} (2σ).
• docs/ui.mjs::drawStrengthCurve — adds a small on-canvas shaded: ±2σ legend in the plot's top-right corner.
• About-modal text already documented ±2σ for the band — kept as-is, now consistent end-to-end.

2. Strict-lint hardening + dev-onboarding (3a7e82f)

CI's flake8 strict-select now covers the full set of bug-detector and key style codes (matching BoTorch/GPyTorch's 88-char convention):

Code	What it catches
E9, F63, F7, F82	syntax errors, undefined names
F401, F811, F841	unused imports / redefinitions / unused locals
E202	whitespace before }
E226	missing whitespace around arithmetic operator
E251	unexpected spaces around keyword/parameter equals
E402	module-level import not at top of file
E501	line too long (88 chars)
E741	ambiguous variable name (I / l / O)

Files modified to conform

• boxcrete/slump_model.py — drop unused import torch.
• docs/generate_mix_analyses.py — drop unused cost/str_1 locals; wrap ~20 long f-strings.
• test/test_partial_fixed_noise_likelihood.py — drop unused n_total.
• boxcrete/features.py — move from boxcrete.utils import DEFAULT_X_COLUMNS to top of module (E402); wrap 3 long lambda-builder lines.
• experiments/regenerate_strength_json.py — relocate # noqa: E402 to opening line of multi-line import; wrap 2 long print lines.
• boxcrete/plotting.py:191 + test/test_models.py:412 — rename I = torch.eye(...) → eye_n (E741: linear-algebra I clashes with 1/l).
• boxcrete/utils.py — f-string {x = } → {x=} (E202/E251); wrap 5 long docstring/error lines.
• experiments/check_artifacts_drift.py — *100 → * 100 (E226).
• test/test_lengthscale_identifiability.py — *100 → * 100 (E226).
• boxcrete/concrete_model.py — wrap 5 long docstring/error lines.
• boxcrete/kernels.py — wrap a 196-char inline pragma: no cover comment.
• boxcrete/likelihoods.py — wrap 4 long pragma: no cover lines.
• boxcrete/slump_model.py — wrap 1 long module-docstring line.
• test/test_strength_curve_monotonicity.py — wrap 3 long error-message lines.
• test/test_models.py:220, test/test_utils.py:679 — tighten 2 long docstrings.

Bonus fix in .flake8

The explicit exclude list (build, dist, .eggs) silently overrode flake8's defaults, so .git, __pycache__, .hg, etc. were being linted. Sapling stores backup copies of working-tree files under .git/sl/origbackups/ — without explicit re-add of the default excludes, flake8 reported 26 phantom violations from old deleted research code. Defaults restored explicitly.

Developer ergonomics

• .pre-commit-config.yaml synced to match CI's strict-select. "Green locally" now implies "green CI" — no more surprises after pushing.

• README.md new ## Development section explaining one-time setup:

  pip install -e ".[dev]"
  pre-commit install