Roadmap: stabilize thread/turn/item protocol across runtime surfaces

[Hmbown]

Summary

The thread/turn/item model is one of the strongest parts of the project, but protocol parity across the TUI runtime, runtime API, app-server, and resume/fork flows is still incomplete. We should stabilize that surface so outside clients can rely on it.

Scope

• Close field and behavior gaps across:
  • TUI runtime
  • runtime HTTP/SSE API
  • app-server transport
  • resume/fork/compact flows
• Preserve structured system prompt data across resume/fork/compaction instead of flattening it to plain text
• Strengthen schema-version and migration handling for persisted runtime records
• Add compatibility tests for the most important thread lifecycle operations

Acceptance Criteria

• Resume/fork/compact behavior is consistent across runtime surfaces
• Structured prompt metadata survives round-trips where intended
• Protocol docs match actual wire behavior
• Tests cover the main lifecycle paths and persisted-state compatibility

Starting Points

• docs/RUNTIME_API.md
• crates/app-server/src/lib.rs
• crates/protocol/src/lib.rs
• crates/tui/src/runtime_api.rs
• crates/tui/src/runtime_threads.rs

[Hmbown]

Closing as stale — these April 11 roadmap items predate the v0.4.x / v0.5.0 work and are being replaced by concrete, actionable issues (#28-#35). Will reopen specific items if/when they're picked up.