fix(cli): reset slash-command conflict dedupe when conflicts reappear

## Summary
`SlashCommandConflictHandler` permanently records every conflict key it has ever notified in `notifiedConflicts`, but it never expires or resets those entries when a conflict disappears and later reappears.

## Evidence
In `packages/cli/src/services/SlashCommandConflictHandler.ts`, `notifiedConflicts` is a process-lifetime `Set`. `handleConflicts(...)` skips any conflict key that has ever been seen before, even if that conflict was resolved earlier in the session.

## Impact
If an extension, workspace command, or MCP prompt conflict is resolved and then reintroduced later, the user may never receive a second notification. The set also grows monotonically over the life of the process.

## Expected behavior
Users should be notified again when a previously-resolved conflict reappears, and the dedupe state should track currently-active conflicts rather than every historical conflict forever.

## Suggested fix
Track active conflict keys per flush or per current conflict state instead of using an unbounded permanent set, and add a regression test for reintroduced conflicts.