feat: Session Lineage — related plans from the same agent session

[Tyru5]

Summary

Adds Session Lineage so opening a plan shows other plans from the same agent session (and parent/child thread links when parentThreadId is present), using metadata adapters already index.

Changes

• New plan-lineage helper: groups by agent + sessionId, links via parentThreadId, soft fallback to same agent + workspace + branch/day
• Plan viewer Same session / Session thread / Nearby in workspace timeline with click-to-navigate
• Quiet session marker on plan list rows that share a sessionId with the same agent
• Caps related peers at 8 around the current plan (avoids unbounded session timelines)
• Wired through OSS and EE dashboards (including split view / Plannotator review)

Review follow-ups

• Matched viewer peer lookup to list badge agent-scoping (${agent}\\0${sessionId})
• Added peer cap with a window centered on the current plan

Test plan

• bun test packages/web/src/client/lib/plan-lineage.test.ts
• bun run lint (0 errors)
• bun run fmt:check
• Manual OSS: list shows session on sibling rows; viewer shows Same session timeline; click navigates between Alpha/Beta

Demo

Plan list session labels

Plan viewer Same session section

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[greptile-apps]

Confidence Score: 5/5

Safe to merge; both previously flagged issues (agent scoping and session peer cap) are addressed in this version.

The new plan-lineage.ts module is well-tested and the agent-scoped session peer lookup is correctly capped at 8 entries. The two remaining notes are non-blocking quality concerns.

The parent/child loops in plan-lineage.ts (lines 164–182) are the only area worth a second look.

_{Reviews (2): Last reviewed commit: "fix: align session lineage peers with li..." | Re-trigger Greptile}