fix(config): make key validation locale-independent

[Math1987]

Summary

• pin both gstack-config key validators to the C locale
• preserve the strict ASCII key contract instead of widening it with a locale-aware character class
• add a cross-platform regression tripwire plus get/set behavior coverage

Fixes #2494

Why

POSIX bracket ranges follow locale collation. With GNU grep under tr_TR.UTF-8, the ASCII letter i can fall outside [a-z], so valid config keys are rejected and skill preambles silently fall back to their defaults.

Tests

• fail-first: the new regression test failed on main because neither validator pinned LC_ALL
• bun test test/gstack-config-key-locale.test.ts test/user-slug-fallback.test.ts test/docs-config-keys.test.ts test/explain-level-config.test.ts test/gstack-config-redact-keys.test.ts — 35 pass, 0 fail
• bash -n bin/gstack-config
• git diff --check

Full-suite note

I also attempted bun test, but it did not complete cleanly in my multi-repository macOS workspace: unrelated existing suites timed out or depended on unavailable local services and fixture assumptions (including global discovery, gbrain fixtures, and local server binding). The focused config suite above is green.

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