fix(agent): restore doot-native task support in Agent API, remove stale test

Two independent pre-existing failures, both from incomplete refactors:

- agent.go's four task write/create switches (complete/uncomplete,
  update due date, update details, create) never got a "doot" case
  added when the Todoist integration was removed (945c345) in favor
  of native tasks -- the test file was updated to use source=doot,
  but the handlers themselves still only recognized "trello"/"gtasks",
  so every native-task Agent API call 400'd with "Unknown source".
  getAtomDetails already had a "doot" case, confirming this was an
  incomplete migration, not an intentional gap.
- TestMealToAtom in atom_test.go tested MealToAtom, a function removed
  from atom.go months earlier (b2d8fc4) when meals were dropped from
  the unified Atom/timeline system; the Meal struct itself is still
  used elsewhere (shopping/meals feature) but no longer produces atoms,
  and the test was never cleaned up to match.

go test ./internal/... ./cmd/... is now fully green with no failures.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01VTUSAEKfsPc6WGDq45yPHD

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