Verify /new and /clear slash commands work end-to-end #124

Problem

/new (archive-and-new) and /clear [N] (context trim) were implemented before PR #108 but have not been tested end-to-end.

What to verify

/clear

• /clear (no argument) dispatches thread.context.trim with no keepLastNTurns
• /clear 3 dispatches thread.context.trim with keepLastNTurns: 3
• Decider emits thread.trim-point-created and thread.session-stop-requested
• Trim point divider appears in UI with correct pruned message count
• Messages before trim point are collapsible/expandable
• Provider session restarts on next turn with correct trim boundary
• Blocked during active turn
• Slash command autocomplete shows /clear and /clear N

/new

• /new dispatches thread.archive-and-new
• Decider emits thread.archived-and-new-created
• New thread inherits worktree, branch, project from archived thread
• Archived thread remains accessible in thread list
• Blocked during active turn
• Slash command autocomplete shows /new

Related

• Parent PRD: PRD: Context Trimming Commands (/clear, /compact, /new) #82
• Spotted during review of feat: provider compaction + context trim + archive-and-new + review flow #108