feat(coding-agent): persist parent edges and derived depth for all sessions

[snimu]

Every session now records two facts in its file on disk: who spawned it (its parent) and how deep it sits in the spawn tree (a root is depth 0, its children are depth 1, and so on).

Until now we could only guess at this from whatever happened to be in memory, which broke down for sessions that weren't loaded. Now it's stored once, at creation, and read back everywhere — session lists, the saved-session catalog, the UI.

Two rules that matter:

• Spawning a child stores parent + 1 as its depth.
• Forking or branching a session copies the original's depth, because a fork of a root is still a root — it's a copy, not a child.

Old session files without these fields still work; we just don't know their depth and fall back to sensible defaults.

---

This is part 1–7 of a stack (each PR builds on the one before it):

1. feat(coding-agent): lazy child session hydration #583 — don't load finished subagents until someone needs them
2. feat(coding-agent): persist parent edges and derived depth for all sessions #584 — record each session's parent and depth on disk
3. feat(coding-agent): /rlm-max-depth session command #585 — new /rlm-max-depth command to control how deep agents can nest
4. feat(coding-agent): recursive scoped agents view #586 — browse the agent tree in the agents view
5. feat(coding-agent): delete child-agent inspector; one subagent summary line #587 — replace the child-agent inspector with one summary line
6. feat(coding-agent): whole-tree idle eviction #588 — shut down worker processes whose sessions have all gone idle
7. feat(coding-agent): intra-tree child session passivation #589 — quietly unload idle child sessions inside a running worker

Tests: the full suite at the top of the stack fails only on tests that already fail on the base branch. Each branch in the stack also passes checks and its own tests independently.

Note

Persist parent edges and derived depth in session headers for all session types

• All new sessions now write rlmDepth to their headers: root sessions default to 0 (or the RLM_DEPTH env var), child sessions increment from their parent's persisted depth.
• Resumed sessions read rlmDepth from their persisted header rather than the environment, so depth is stable across restarts.
• Fork, branch, and materialize paths carry the source rlmDepth forward consistently.
• Session listings (active and inactive) now include rlmDepth, and daemon protocol is bumped to schema revision 10.
• Serialization/deserialization of SavedSessionInfo is consolidated into a new shared saved-session-info.ts module.
• Risk: invalid RLM_DEPTH env values now throw at root session creation instead of being silently ignored.

Changes since #584 opened

• Changed SessionInfo.rlmDepth from optional to required and introduced resolveSessionRlmDepth utility function that computes depth from persisted header value, defaults to 0 for root sessions, or infers depth by counting nested sub-* directory segments in the session file path [6a1950b]
• Updated all session creation and loading code paths in SessionManager to compute and persist rlmDepth using resolveSessionRlmDepth, including scanSessionInfo, open, constructor existing-file handling, materializeSessionFile, createBranchedSession, and forkFrom [6a1950b]
• Updated AgentDaemon.buildPassiveRlmSubagentRequest to rely solely on SessionInfo.rlmDepth and modified AgentDaemon.createRuntime to infer rlmDepth using resolveSessionRlmDepth when resuming from existing session files [6a1950b]
• Updated test helpers in agents-view-state.test.ts, daemon-session-list.test.ts, and their makeSessionInfo functions to default rlmDepth to 0, and added comprehensive test coverage in file-operations.test.ts and daemon-mode.test.ts validating depth inference from path structure [6a1950b]
• Modified resolveSessionRlmDepth utility to derive child depth from parent session header when available and refined path-based depth inference [08c8870]
• Updated AgentDaemon.buildSessionListWithPassiveRlmSubagents method to prefer registry entry depth over session info depth [08c8870]
• Added test coverage for depth resolution precedence and path-based inference behavior [08c8870]
• Implemented recursive parent-based session depth resolution with cycle detection and selective depth increment [748b47c]
• Added test coverage for depth backfilling behavior in legacy forked sessions [748b47c]
• Modified resolveLegacySessionRlmDepth function in coding-agent to resolve parent session paths relative to each session file's directory [121ae3d]
• Added test case validating relative parent path resolution in legacy session depth calculation [121ae3d]

^{Macroscope summarized 842927d.}

---

Note

Medium Risk
Changes session header semantics, fork/branch depth rules, and daemon wire schema; invalid RLM_DEPTH now throws at root session creation, and legacy depth inference could mis-rank edge-case trees.

Overview
Persists RLM spawn depth (rlmDepth) in session file headers and threads it through spawn, fork, branch, and resume so nesting limits no longer depend on in-memory state or RLM_DEPTH alone.

New and child sessions write parentSession + rlmDepth at creation (children get parent depth + 1; forks/branches copy the source depth). AgentSession resolves depth from config → persisted header → env, and exposes rlmDepth. resolveSessionRlmDepth backfills legacy files (parent chain, sub-* path segments, open-time rewrite) and makes SessionInfo.rlmDepth required for listings.

Daemon / UI: saved-session and session-list DTOs include rlmDepth (schema revision 10); passive subagent rows use registry depth when present; serialization moves to saved-session-info.ts. Subagent runtime creation passes explicit rlmDepth into newSession.

^{Reviewed by Cursor Bugbot for commit 121ae3d. Bugbot is set up for automated code reviews on this repo. Configure here.}

[cursor]

Cursor Bugbot has reviewed your changes and found 1 potential issue.

^{❌ Bugbot Autofix is OFF. To automatically fix reported issues with cloud agents, enable autofix in the Cursor dashboard.}

^{Reviewed by Cursor Bugbot for commit 08c8870. Configure here.}