feat(lint-org): add four structural heading checkers org-lint misses

org-lint validates links, drawers, blocks, and babel, but not heading well-formedness. These four catch hand-edit defects it stays silent on: an indented heading demoted to body text (the task vanishes from the agenda and never archives), bare stars with no title, a malformed priority cookie org rejected, and a level-2 DONE/CANCELLED with no CLOSED line. All judgment-only and regex-based, wired in after the existing dated-header check. The last one pairs with the new aging step, which archives an undated completed task immediately.

I tightened the indented-heading check to two-or-more stars. The proposed one-or-more-stars regex flagged indented single-star lines, but an indented single * is a valid plain-list bullet, not a lost heading, so it false-positived on legitimate lists (confirmed: three valid bullets flagged). A ** is never a bullet, so an indented one is unambiguously a demoted heading. Added a test that a single-star list stays silent.

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