Session chain links break on force-kill; sidechain/subagent sessions clutter resume list

Problem

Two issues with the session resume list that are not covered by the sessions-index.json desync bugs (#22205, #22114):

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1. Session chain links break permanently when parent is force-killed

Sessions are chained via parentUuid → leafUuid (written in the summary entry). When a parent session is force-killed before writing a summary, its leafUuid is never persisted, permanently breaking the chain for any child session that was resumed from it.

Data from a real project (322 sessions):

• 43 sessions have a parentUuid (are continuations of a prior session)
• Only 19 resolve to a parent session (44% success rate)
• 24 sessions are permanently orphaned — their parentUuid points to a leafUuid that was never written

Root cause: The summary entry (which contains leafUuid) is only written on clean exit. Force-kill (terminal closed, system freeze, kill -9) means no summary → no leafUuid → broken chain.

Suggested fixes:

• Write summary/leafUuid on SIGINT/SIGTERM before exiting
• Write intermediate summaries more frequently (already happens during long sessions, but not reliably at the end)
• When resolving chains, fall back to matching by sessionId if leafUuid lookup fails
• Show orphaned sessions as standalone roots rather than hiding them

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2. Sidechain/subagent sessions clutter the resume list

Plugin-spawned sessions (MCP observers, subagents, warmup sessions) have isSidechain: true and filenames prefixed with agent-, but they still appear in the resume list alongside real user sessions.

Data from same project:

• 192 of 322 sessions (59%) are sidechains (isSidechain: true, agent- prefix)
• 34 of those are just "Warmup" messages with no useful content
• These make it difficult to find actual work sessions in the list

Suggested fix:

• Filter sessions with isSidechain: true or agent- prefixed IDs from the default /resume list
• Optionally show them behind a flag like --all or in a separate section

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Environment

• Claude Code CLI (Opus 4.5)
• macOS Darwin 23.6.0
• 322 sessions in project directory (130 user, 192 sidechain)
• Plugins: claude-mem (spawns observer sidechains via hooks)

Related

• #22205 and #22114 cover the sessions-index.json desync issue (sessions missing from index entirely). This issue is about chain integrity and list filtering, which are separate problems.