feat: reject digit-adjacent tokens and raise default threshold to 8

[larkiny]

Summary

• Digit rejection in stripToken — Tokens where the stripped leading/trailing portion contains digits (e.g. board[0], abandon123, 3abandon) are now disqualified as sequence-breaking (null), preventing false positives from array indexing and code constructs. Applied to TypeScript and both Bash scripts.
• Default threshold raised from 5 → 8 — Exported as DEFAULT_THRESHOLD from detect.ts and shared across check-staged.ts and scan-repo.ts. Reduces false positives from prose containing common BIP39 words while still catching 12/24/25-word mnemonics.
• README updated — All threshold references updated from 5 to 8; config example now shows BIP39_THRESHOLD=12 as the override illustration.

Test plan

• All 122 existing tests pass (pnpm vitest run)
• New tests for digit-prefixed, digit-suffixed, and array-indexed tokens
• New test for Python board[0]..board[8] code snippet (no false positive)
• All test data updated to use 8-word BIP39 sequences matching new threshold
• DEFAULT_THRESHOLD constant verified in detect.ts, check-staged.ts, scan-repo.ts

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

Files with missing lines	Coverage Δ
ts/check-staged.ts	90.80% <100.00%> (ø)
ts/detect.ts	92.96% <100.00%> (+0.16%)	⬆️
ts/scan-repo.ts	85.02% <100.00%> (ø)

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