fix(daemon): drop deleted rlm children's kernel state and dedupe artifact paths

[snimu]

What this fixes

A subagent child's state is split across two stores: its transcript dir (session-artifacts/<parentId>/sub-XXXX/ — transcript plus the rlm-subagent.json display file) and a nested artifact tree (session-artifacts/<parentId>/session-artifacts/<child-uuid>/) where the child's own session puts kernel snapshots (kernel-state.dill) and harness state. Deleting a child never touched the nested tree, so every deleted child leaked its kernel snapshots forever — unreachable state, since a deleted child has no revival path.

Policy, stated explicitly

Deleted subagents are deleted. On child deletion, kernel snapshots and harness state go. Only the transcript .jsonl and the display-file tombstone remain, preserving the documented "deletion keeps the transcript readable" contract. The boundary: transcript + display file = durable record, retained; nested artifact dir = runtime cache, dropped.

What it does

• Deletion drops the cache through the existing single code path. deleteSessionArtifacts (which full-session delete already used) is now also invoked by rlm child deletion, after the display tombstone and the ledger delete are durable, best-effort — cache cleanup can never fail a deletion. A retried deletion re-sweeps (crash-window heal). Because a resident child's kernel dispose flushes a final snapshot during teardown (which would resurrect the dir), the teardown paths re-sweep in finally blocks, so even a throwing teardown cannot skip the sweep; scheduled-job cancellation during deletion is likewise best-effort — a jobs-store error is logged rather than failing a deletion whose tombstones are already durable (deliberate semantic change).
• Path dedup. The artifact-path formula (join(dirname(dirname(sessionFile)), "session-artifacts", id)) was hand-duplicated in 7 places across session-manager, session-file-actions, catalog process, supervisor, daemon-mode, and the spawn ledger. There is now one exported canonical helper family in session-manager (getSessionArtifactsRoot / getSessionArtifactPath / getSessionArtifactPathForFile) and every site uses it — zero copies of the formula remain outside the helpers.
• Degenerate-name guard. deleteSessionArtifacts refuses a session file whose basename yields an empty id (e.g. literally .jsonl) — unguarded, the path join would resolve to the artifacts root itself. All four daemon sweep call sites route through this single guarded rm.

Scope note

Artifact dirs already leaked by pre-fix builds are deliberately NOT cleaned up by this PR (an earlier draft carried an opportunistic reaper; it was cut — a permanently-running mechanism is the wrong trade for a one-time mess). New children stop leaking by construction. Existing garbage can be removed manually (for each tombstoned child, remove session-artifacts/<parentId>/session-artifacts/<child-uuid>/) or by a future doctor --fix.

Numbers

Net src +49 (7 files, +90/−41): the helper family and the deletion hook, minus the deduplicated formulas.

Testing

Deletion removes the artifact dir while transcript, display tombstone, and a grandchild's transcript survive; teardown-throw still sweeps (regression, fails pre-fix); throwing jobs-cancel doesn't fail deletion (regression); rm failure is best-effort; degenerate-basename guard (proven to fail unguarded); helper path-equivalence. Targeted daemon suites green, npm run check clean.

Note

Drop deleted RLM children's kernel state and deduplicate artifact path resolution

• When an RLM subagent is deleted, the daemon now sweeps its nested artifact directory via a new deleteRlmSubagentArtifacts method; cleanup is best-effort and never fails the parent deletion flow.
• Artifact directory removal is wrapped in try/finally so it runs even when child teardown or scheduled-job cancellation throws.
• Tombstoned-edge retry paths now also sweep leftover artifact dirs for each tombstoned child.
• Centralizes artifact path computation into three new helpers (getSessionArtifactsRoot, getSessionArtifactPath, getSessionArtifactPathForFile) in session-manager.ts, replacing scattered manual dirname/join calls across daemon, supervisor, ledger, and cron-job modules.
• deleteSessionArtifacts gains a safety guard that no-ops for degenerate .jsonl-only basenames to prevent accidental removal of the artifacts root.

^{Macroscope summarized 8fcb0bc.}