Agent Runtime: Session fork feature is stubbed but not fully implemented

[wangtsiao]

Describe the bug

Session forking has a stub/placeholder implementation but is not yet functional.

Current state

crates/core/src/fork.rs contains the fork admission logic and inherited segment construction, but several fields are placeholders:

• source_parent_session_id: SessionId::new() — uses a new random ID instead of the actual parent
• source_range: SegmentSourceRange { start_offset: 0, end_offset: 0 } — always zero
• segment_hash: String::new() — empty hash

The fork is not exposed via any Desktop or TUI UI, and the full replay-with-independent-segments flow described in the module's doc comment is not wired up.

Expected behavior

• Users can fork a session from the Desktop app (at a specific turn)
• Forked sessions retain inherited history correctly
• Independent segment loading works for forked sessions

Related component

• crates/core/src/fork.rs
• crates/protocol/src/acp_session.rs