Hide paused chains' tasks from the task list

Pausing a chain is supposed to take its work off the list -- that's the
entire point of the control. But the three task-fetching queries only
ever checked `chain_unlocked`, never the owning chain's status, so a
paused chain's head task kept appearing exactly like an active one.
Verified against production: 4 of the 8 visible chain tasks came from
chains that had been explicitly paused ("Will", "ABYC Marine Electrical
certification", "Standard services agreement", "Find a broker who writes
marine trades"). Half the chain clutter on the list was work the user
had already told doot to put away.

Fixed the class, not just the instances: the predicate now lives in one
shared visibleChainTaskClause constant used by all three queries, with a
structural test asserting no query spells it out inline and that every
query still references it. The previous shape -- the same predicate
copy-pasted into three separate query strings -- is exactly why a fourth
query would have gotten it wrong too.

Also added task_chains to sqlite_test.go's leaner test schema, since the
queries now join against it even for chain-less tasks.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_018AwnALZL6bD4VBtATDVv3B

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