lint: enable PLW1514 as a blocking ruff rule

Turns the existing 'all lints disabled' stance into 'exactly one lint
enabled' — PLW1514 (unspecified-encoding) catches bare open() /
read_text() / write_text() calls that default to locale encoding on
Windows (cp1252), silently corrupting non-ASCII content.

Changes:

1. pyproject.toml
   - Migrate [tool.ruff] top-level select → [tool.ruff.lint].select
     (deprecated config location, ruff was warning on every run)
   - Add preview = true (PLW1514 is a preview rule in ruff 0.15.x)
   - select = ['PLW1514'] (exactly one rule, deliberately minimal)
   - per-file-ignores exempt tests/, plugins/, skills/, optional-skills/ —
     those have their own conventions or intentionally exercise edge cases

2. website/scripts/extract-skills.py
   - Fix 3 remaining bare opens (website/ was excluded from the main
     sweep but needed for ruff check . to go green)

3. tests/test_lint_config.py (new, 5 tests)
   - Guards against accidental rule removal.  If someone deletes PLW1514
     from the select list or disables preview mode, these tests fail
     with a loud message explaining why the rule exists.

Paired with a companion commit (held locally for now, pending a token
with workflow scope) that adds a blocking ruff step to .github/workflows/
lint.yml.  Without that companion commit, ruff is configured correctly
but nothing in CI enforces it yet — the advisory PR comment will still
surface new PLW1514 violations though, so authors see them.

Verified: ruff check . → exit 0, 0 violations across the repo.
Test suite: 90 passed, 14 skipped, 0 failed.

pyproject.toml

@@ -188,7 +188,25 @@ exclude = ["tinker-atropos"]
 
 [tool.ruff]
 exclude = ["tinker-atropos"]
-select = [] # disable all lints for now, until we've wrangled typechecks a bit more :3
+preview = true  # required for PLW1514 (unspecified-encoding) — preview rule
+
+[tool.ruff.lint]
+# All other lints are intentionally disabled (see comment history on this
+# file) while we wrangle typechecks — but PLW1514 is too load-bearing to
+# keep off.  Bare open()/read_text()/write_text() in text mode defaults to
+# the system locale encoding on Windows (cp1252 on US-locale installs),
+# which silently corrupts any non-ASCII file content.  We had three
+# separate Windows sandbox regressions in one debug session before
+# adding the explicit encoding.  This rule keeps new code honest.
+select = ["PLW1514"]
+
+[tool.ruff.lint.per-file-ignores]
+# Tests can intentionally exercise locale-encoding edge cases.
+"tests/**" = ["PLW1514"]
+# Skills and plugins are partially user-authored — their own conventions.
+"skills/**" = ["PLW1514"]
+"optional-skills/**" = ["PLW1514"]
+"plugins/**" = ["PLW1514"]
 
 [tool.uv]
 exclude-newer = "7 days"