list: render as a single chronological timeline (Phase B) - #30

list's default view moves from Task/Memo sections to one
chronological list (log.Timeline merges and sorts both, replacing
the split-then-render path for this case). Each line now leads with
its creation time and a marker ("・" for a memo, "[ ]"/"[x]"/"[>]"
for a task), with the hash and tags trailing in parens — memos keep
their hash since del/tag still need it. view.ItemList is removed;
Timeline is its only remaining caller. -t tag filtering and all other
list modes (-a, -s, a specific date) are unaffected.

- Split/Timeline: sort.Slice -> sort.SliceStable so items sharing the
  same millisecond CreatedAt keep a deterministic order instead of an
  unspecified one.
- openTaskHashes: match the "[ ]" marker only at its fixed position
  and read the hash from the last parenthesized group, so item
  content containing "[ ]" or literal parentheses no longer produces
  a false match.

Found in Phase B code review; findings 4-6 from the same review are
documented in docs/reports/2026-08-08-phase-b-code-review.md and left
open for Phase C.

Add model.Item.Status(), giving Closed precedence over Started, so
the memo/open/started/closed precedence lives with the data instead
of being re-derived inline in view.Timeline. view.Timeline now maps
Status() to a marker with a flat switch instead of an if-guarded one.

Addresses findings 5-6 from the Phase B code review (already
documented as deferred in docs/reports/2026-08-08-phase-b-code-review.md,
now updated to reflect the fix).

- openTaskHashes: rename open/close locals so they don't shadow the
  close builtin.
- model.go: add a doc comment on the Status const block so exported
  constants aren't flagged as undocumented.