feat(todo): focus session-chain todo cards

[yujiezhang-ops]

Summary

• Build Todo cards around session task chains and merge low-signal continue/retry turns into the relevant chain.
• Hide legacy/noisy completed backlog cards by default while keeping search and folded access.
• Persist Todo extraction status so reloads show running/done/error accurately and stale running states become retryable.

Test Plan

• npm run pre-pr
• Chrome verified http://localhost:3114/#actions after service restart