Feature request: Session chaining for cross-session context continuity

Problem

When a conversation spans multiple sessions (due to auto-splitting, crashes, or terminal restarts), --resume only restores the context of a single session. The logical thread's earlier context — motivations, reasoning chains, and decisions that led to the current state — is lost.

Example scenario:

1. Session 1: Discuss team/org challenges → develop strategy direction
2. Session 2: Analyze contradictions in strategy v3 → build decision tree
3. Session 3: Finalize strategy v4

Resuming Session 3 only recovers the v4 finalization context. The why behind the strategy (from Session 1) is gone, making follow-up discussions shallow.

Current workarounds

• Writing handoff notes to files between sessions — but this consumes tokens and adds overhead
• Using persistent memory — but only captures summaries, not the full reasoning chain
• --continue / --resume — useful but limited to single-session context

Proposed solution

Some form of session chaining or thread-level history, where:

• Sessions that are logically connected (e.g., auto-split from the same conversation, or manually linked) can share context
• Resuming a session could optionally load prior linked sessions' context (perhaps compressed)
• Users can group sessions into named threads

Impact

Power users who run long, multi-session workflows (strategy development, complex debugging, multi-day projects) lose significant context at every session boundary. This forces either redundant re-explanation or file-based workarounds that increase token consumption.