chore(trace): remove unused DiscoverSession and SessionDirForRepo from the Provider interface #3536

Problem

The trace.Provider interface in app/cli/internal/trace/provider.go has two methods that no production code calls through the interface:

• DiscoverSession. Only two unit tests call it: claude/session_test.go and opencode/session_test.go.
• SessionDirForRepo. Only the Cursor provider calls its own implementation, from CopySessionData (cursor/provider.go). The Claude implementation is not called. The OpenCode implementation returns "".

Each new provider must still implement both methods. For example, the Pi provider in #3523 needs them only to satisfy the interface.

Proposal

1. Remove DiscoverSession and SessionDirForRepo from the Provider interface.
2. Remove the DiscoverSession implementations, the DiscoveredSession type, and their tests.
3. Remove the Claude and OpenCode SessionDirForRepo implementations.
4. In the Cursor provider, make CopySessionData call transcriptDirForRepo directly, and remove the wrapper.

Out of scope

• A pre-push scan that finds sessions whose hooks did not run. If a later design needs one, it can add a new method for it.