docs: restructure TASKS.md into a catalog — one file per task in TASKS/
TASKS.md is now the index (status table, open items, research pointer). 33 files: 32 task files + 1 research facts file. The full take-saga narrative and all design decisions are preserved verbatim; the catalog makes the queue readable without opening every task body. Schema and AGENTS.md updated to reflect the new layout.
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