Reader follow-ups for execution-graph: Codex session_meta fork/continuation, Claude system-event tool-result capture, Claude line-level fork/continuation

[willwashburn]

Context

PR #77 landed the execution-graph substrate (#42). Subsequent granular PRs populated it:

• PR Populate execution-graph records from OpenCode parser (#93) #119 (OpenCode passive reader: populate SessionRelationshipRecord and ToolResultEventRecord #93) — OpenCode root + subagent relationships and tool-result events.
• PR Populate execution-graph records from Codex parser (#87) #121 (Codex passive reader: populate SessionRelationshipRecord and ToolResultEventRecord #87) — Codex root + subagent relationships and tool-result events.
• PR Emit fork and continuation relationships from Claude parser (#112) #122 (Claude reader: populate fork and continuation SessionRelationshipRecord rows #112) — Claude fork/continuation rows from cross-file `parentUuid` + `/resume` markers.

PR #126 ("Complete passive execution graph coverage", recommended for close) attempted three pieces beyond what those granular PRs landed. This issue tracks porting those three pieces.

Proposal

A. Codex: fork / continuation rows from `session_meta`

In `packages/reader/src/codex.ts`, read `session_meta.payload` for:

• `forkSessionId` → emit a `SessionRelationshipRecord` with `relationshipType: 'fork'` and `relatedSessionId` set.
• `continuedFromSessionId` → emit a `SessionRelationshipRecord` with `relationshipType: 'continuation'` and `relatedSessionId` set.
• `sourceSessionId` → populate the `sourceSessionId` field on emitted rows (mirror Claude's behavior in PR Emit fork and continuation relationships from Claude parser (#112) #122).
• `cli_version` → populate `sourceVersion`.

PR #121 only emitted `root` + `subagent` for Codex. The `session_meta` payload is the only evidence channel Codex exposes for fork/continuation.

B. Claude: system-event tool-result capture

In `packages/reader/src/claude.ts`, capture `type: 'system'` records carrying `subagent_session_id` / `agent_id` / `parent_tool_use_id` as additional `ToolResultEventRecord` rows with `eventSource: 'subagent_notification'` (or similar — match the existing event-source vocabulary from PR #77).

PR #122 only handled `system` records for compaction boundaries and `/resume` markers — subagent system events were skipped.

C. Claude: line-level explicit fork / continuation

In `packages/reader/src/claude.ts`, in addition to the existing `parentUuid` cross-file detection + `/resume` markers from PR #122, read explicit `forkSessionId` / `continuedFromSessionId` fields from per-line user/assistant records (where present in the JSONL) and emit the corresponding `fork` / `continuation` rows directly. Should compose cleanly with the existing `reconcileClaudeSessionRelationships` pass.

Acceptance criteria

• Codex fixtures with `session_meta.payload.forkSessionId` / `continuedFromSessionId` produce `fork` / `continuation` rows; `sourceVersion` is populated from `cli_version`.
• Claude fixtures with `type: 'system'` records carrying subagent identifiers produce additional `ToolResultEventRecord`s without disturbing existing compaction / `/resume` handling.
• Claude fixtures with explicit `forkSessionId` / `continuedFromSessionId` line-level fields produce the corresponding rows; reconciliation with cross-file inference stays idempotent.
• Tests in `packages/reader/src/codex.test.ts` and `packages/reader/src/claude.test.ts` cover each path.

Out of scope

• OpenCode equivalents — OpenCode storage doesn't surface fork/continuation evidence (per Claude reader: populate fork and continuation SessionRelationshipRecord rows #112's out-of-scope note).
• Reader-side performance optimizations.
• Consumer CLI surfaces (tracked separately).

Refs

• Spun out of Execution graph for passive readers: session relationships and tool-result event chronology #42 (PR Complete passive execution graph coverage #126, recommended for close)
• Reference implementation: PR Complete passive execution graph coverage #126's diffs to `packages/reader/src/codex.ts` and `packages/reader/src/claude.ts`
• Predecessors: PRs Populate execution-graph records from OpenCode parser (#93) #119 (OpenCode passive reader: populate SessionRelationshipRecord and ToolResultEventRecord #93), Populate execution-graph records from Codex parser (#87) #121 (Codex passive reader: populate SessionRelationshipRecord and ToolResultEventRecord #87), Emit fork and continuation relationships from Claude parser (#112) #122 (Claude reader: populate fork and continuation SessionRelationshipRecord rows #112)