feat: import existing sessions and restore Codex runtime UI

[duxiaoxiong]

Summary

This draft PR adds import and runtime restoration support for existing agent sessions, with a focus on Codex parity in HAPI's web UI.

• Import existing Codex/Claude sessions from local transcripts and mark already-imported sessions.
• Preserve Codex runtime config when importing/resuming, including model, reasoning effort, permission mode, collaboration mode, and service tier when available.
• Surface Codex runtime config in session summaries/status UI.
• Restore Codex subagent display for spawn/wait flows, including multiple concurrent subagents, nickname propagation, and lifecycle status updates.
• Hide HAPI title-tool plumbing from visible chat while allowing Codex/native title updates to update session metadata.
• Improve subagent preview cards so the nickname is primary, with stable per-agent accent colors.

Why

Imported Codex sessions previously restored message history but lost the active runtime configuration, so continuing an imported session could silently fall back to default reasoning/model behavior. Codex subagent events also arrived in a different shape than classic tool forwarding, so multi-agent spawn/wait output could show stale or incomplete UI state.

Validation

• bun run typecheck
• bun run test
• bun run build
• Browser smoke test against a temporary local HAPI runtime:
  • created a Codex session
  • spawned two Codex child agents
  • waited for both to complete
  • verified both subagent cards showed nickname-first UI and Completed status

Notes

This is a draft PR because the diff spans import plumbing, runtime config propagation, and Codex subagent UI. Happy to split or trim scope if preferred.