feat(todo-cleanup): add --archive-done mode with ERT test suite

--archive-done moves every level-2 subtree whose TODO state is DONE or CANCELLED out of the "Open Work" section into the "Resolved" section of the same org file, subtree intact. Sections match on a unique level-1 heading containing "Open Work" (case-insensitive) and one containing "Resolved"; a missing or ambiguous section skips the file with a message rather than crashing. Only direct level-2 children move. A DONE entry nested under an open parent stays put. Opt-in, never run by default, doesn't also run the hygiene passes; --check previews without writing.

The CLI dispatch moved into tc-main behind a guard so the new ERT suite can require the file without firing it. Hygiene mode is unchanged.

13 ERT cases (the repo's first elisp tests) cover the move and the stay-put cases, EOF with no final newline, missing or ambiguous sections, lowercase headings, idempotency, and --check. tests/fixtures/todo-sample.org is the synthetic sample, and the Makefile test target now runs the ERT suites alongside pytest.

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