feat: add secret redaction

[CoderDeltaLAN]

Adds a minimal redaction helper for secret-like values.

Scope:

• add dependency-free redaction module;
• redact OpenAI-like keys;
• redact GitHub-like tokens;
• redact AWS access-key-like values;
• redact private-key-like blocks;
• add tests that build synthetic secret-like values at runtime;
• avoid committing raw secret-like fixture values.

Not included:

• no CLI integration;
• no diagnostic finding generation;
• no JSON output;
• no Markdown output;
• no file content analysis;
• no network behavior;
• no CI changes.