a live task can be edited, a resolved one cannot (V-509)

SetTaskStatus was the only mutation on a task row, so a typo in a dictated
task was permanent and a deadline could not move. EditTask rewrites the three
fields capture set — text, due date and weight — and nothing else. Status
stays the one-way ladder SetTaskStatus owns.

Two things the task asked to settle.

A text edit re-normalises the dedupe key and can collide with another live
row. That is ErrTaskDuplicate, a refusal rather than a merge: two live rows
carry two provenances, two capture times and possibly two external
identities, and merging picks a winner for all three with nobody asked. The
surface names the row that holds the text.

A resolved task is refused outright (ErrTaskResolved). Its text is the record
of what was finished, and rewriting it rewrites history.

due nil clears the date, because clearing has to be sayable — an absent date
and "remove the date" cannot be one argument.

internal/ipc/coreapi.go

@@ -142,6 +142,11 @@ type TaskAPI interface {
 	// SetTaskStatus moves a task forward once: candidate→open|dropped,
 	// open→done|dropped. Any other move is refused.
 	SetTaskStatus(ctx context.Context, id int64, status string, ts time.Time, by string) error
+	// EditTask rewrites the three fields capture set: text, due date and
+	// weight. Status is not among them — that ladder is one-way and belongs to
+	// SetTaskStatus. A resolved task is refused, and a text edit that would
+	// duplicate another live task is refused rather than merged.
+	EditTask(ctx context.Context, id int64, text string, due *time.Time, weight int) error
 	// SetTaskFields writes the definition of done and the blocker. Not a
 	// status move, so it is not one-way: he may sharpen a criterion, and a
 	// blocker clears when the person answers. blockedOn is a canonical Nexus