feat: opt-in engine extension points for non-wiki markdown trees

[SunsetDrifter]

Summary

Adds nine opt-in config keys to the shared wikilint/ engine so it can lint markdown trees that aren't wikis (a findings folder, a labs journal) without forking. Every knob defaults to the original behavior — the four variants are byte-for-byte unaffected in what they check.

• markdown_links — relative [text](target) links must resolve on disk (file or directory); inline code spans ignored; resolved page targets count toward inbound links for orphan detection
• orphans — gate to disable orphan warnings + unlinked-mention hints
• index_file / index_body_fn — relocate the generated index and/or replace the body generator with a callable; the index is skipped by page discovery when it lives inside a page dir
• iso_date_fields — which frontmatter fields get ISO-date validation
• extra_secret_patterns / secret_allow_res — extend the secrets scan; suppress matches on allowlisted lines (template placeholders like ${var}, redaction markers). scan_text_for_secrets exposed for reuse by variant extra_checks
• log_file — None disables the log check; same gate added for taxonomy_file and inbox_dir (fixing a latent TypeError when inbox_dir is None)
• extra_checks — callable(pages, report, root) list appended to every check pass
• non_page_allowed entries are now fnmatch patterns (exact names still match)

Each variant's lint.py documents the new keys inline; README gains one Design bullet.

Test plan

• 42/42 tests pass (python3 -m unittest discover -s tests), including the byte-identity and config-key-parity invariants across variants
• 11 new cases covering each extension point (file-accurate line numbers, discovery skip for a relocated index, allowlist suppression, None-gates, glob layout)
• CI runs the same suite on this PR