fix(today): exclude completed tasks by default, add --include-completed

[ryanlewis]

What

things today now returns only open tasks by default. Completed/cancelled items that Things keeps on screen until it logs them out of Today are available behind a new opt-in flag:

things today                      # open tasks only (new default)
things today --include-completed  # + completed/cancelled not yet logged (UI-parity)

Closes #106.

Why

The today view kept completed/cancelled items visible until Things logged them out of Today (UI-parity, added in #74). For automation this over-reports: "in today" stopped meaning "still to do", and consumers had to filter on the bare status: 3 themselves. Defaulting to open tasks is the least-surprising behaviour; --include-completed restores the UI-parity view for anyone who wants it.

How

• internal/db: TaskFilter.IncludeCompleted + a todayWhere(includeCompleted) helper. Default today filters status = 0; with the flag it keeps status IN (2,3) items whose stopDate post-dates manualLogDate (the same logged-out signal as before).
• cmd/things: --include-completed flag. It only affects today, so it's rejected with an error on any other view (including when a trailing project name promotes today → project), matching how --on/--from/--to reject unsupported views — no silent no-ops.
• internal/skill/SKILL.md: documented the new default and flag.

Tests

• DB: default excludes completed and cancelled; --include-completed surfaces unlogged completed + cancelled items pre-log, then drops them once manualLogDate advances.
• CLI: flag parses; rejected on non-today views and on the promoted-project path.

Review

Ran /code-review --fix (xhigh). Applied: the non-today rejection, a start=1 cancelled (status=2) fixture closing a coverage gap, and month-boundary-safe date arithmetic in the test helper.

Knowingly not changed (out of scope / pre-existing):

• --on/--from/--to filter --include-completed items by startDate, not completion date — consistent date-filter semantics across all views.
• TMSettings LIMIT 1 (no ORDER BY) and the NULL stopDate three-valued-logic drop are carried over verbatim from the prior inline clause; not introduced here.