Use reflexive wording for self-assignment timeline events (#8286)

* Initial plan

* Add special wording for self-assignment timeline events

When actor and assignee/unassignee are the same person:
- Use "assigned themselves" instead of "assigned {user}"
- Use "removed their assignment" instead of "unassigned {user}"

This aligns the wording with GitHub.com behavior.

Co-authored-by: alexr00 <38270282+alexr00@users.noreply.github.com>

* Handle self-assignment in mixed assignment/unassignment events

Also apply special wording for self-assignment when both assignments
and unassignments occur in the same consolidated event.

Co-authored-by: alexr00 <38270282+alexr00@users.noreply.github.com>

* Refactor to reduce code duplication in AssignUnassignEventView

Extract joinedAssignees and joinedUnassignees into variables to avoid
repeating the joinWithAnd expressions and improve maintainability.

Co-authored-by: alexr00 <38270282+alexr00@users.noreply.github.com>

* Better match GitHub's language

---------

Co-authored-by: copilot-swe-agent[bot] <198982749+Copilot@users.noreply.github.com>
Co-authored-by: alexr00 <38270282+alexr00@users.noreply.github.com>

Author: Copilot

webviews/components/timeline.tsx

@@ -459,13 +459,20 @@ const AssignUnassignEventView = ({ event }: { event: AssignEvent | UnassignEvent
 	const joinedAssignees = joinWithAnd(assignees.map(a => <AuthorLink key={`${a.id}a`} for={a} />));
 	const joinedUnassignees = joinWithAnd(unassignees.map(a => <AuthorLink key={`${a.id}u`} for={a} />));
 
+	// Check if actor is assigning/unassigning themselves
+	const isSelfAssign = assignees.length === 1 && assignees[0].login === actor.login;
+	const isSelfUnassign = unassignees.length === 1 && unassignees[0].login === actor.login;
+
 	let message: JSX.Element;
 	if (assignees.length > 0 && unassignees.length > 0) {
-		message = <>assigned {joinedAssignees} and unassigned {joinedUnassignees}</>;
+		// Handle mixed case with potential self-assignment
+		const assignMessage = isSelfAssign ? <>self-assigned this</> : <>assigned {joinedAssignees}</>;
+		const unassignMessage = isSelfUnassign ? <>removed their assignment</> : <>unassigned {joinedUnassignees}</>;
+		message = <>{assignMessage} and {unassignMessage}</>;
 	} else if (assignees.length > 0) {
-		message = <>assigned {joinedAssignees}</>;
+		message = isSelfAssign ? <>self-assigned this</> : <>assigned {joinedAssignees}</>;
 	} else {
-		message = <>unassigned {joinedUnassignees}</>;
+		message = isSelfUnassign ? <>removed their assignment</> : <>unassigned {joinedUnassignees}</>;
 	}
 
 	return (