Harden regex initialization and avoid silent ask.md read failures

[kimdzhekhon]

Summary\nTwo hardening issues found:\n1) regex helper functions still use silent fallbacks to with , hiding invalid-pattern failures.\n2) reads existing ask.md using , which swallows IO errors and can lose prior Q&A history.\n\n### Evidence\n- src/input.rs:62\n- src/requirements.rs:64\n- src/main.rs:203\n- src/checks.rs:610, 618, 626\n- src/main.rs:476\n\n### Proposal\nInitialize regexes as explicit fail-fast constants (e.g., ) and change ask.md read to propagate read errors for existing files while treating missing file as empty.