[G1] First-class fork / resume / continuation relationships with cross-file reconciliation (Claude and Codex) #170

Part of #160 (group 1 — independent).

Problem

session_relationships models delegation only. RelationshipType is 'delegated' (sdk-ts/src/contracts.ts:220); the only other relationship value written is materialized_local (Claude remote→local, crates/ai-hist/src/lib.rs:2887-2907). Resumed, forked, and continued sessions are stored as unrelated sessions, so a consumer cannot roll a /resumed conversation up to its origin, and burn's continuity analytics (burn flow rail continuity, summary --by-relationship) have nothing to read.

burn detects all of these today in crates/relayburn-sdk/src/reader/claude/relationships.rs and emits SessionRelationshipRecord { relationship_type: root|continuation|fork|subagent, related_session_id, source_session_id, source_version, ts } (reader/types.rs:497-520):

• Explicit fields on Claude records: forkSessionId, continuedFromSessionId, sourceSessionId (relationships.rs:45-114).
• /resume and /continue markers in user text → local continuation (relationships.rs:259-321; fixture resume-marker.jsonl).
• Cross-file reconciliation (relationships.rs:405-522): builds a uuid → file_session_id map across all parsed files; a file whose first_parent_uuid belongs to another file is a continuation; ≥2 sessions sharing a foreign sourceSessionId are forks (fixtures cross-file-parent, original-session, fork-branch-a, fork-branch-b, explicit-line-relationships).
• Evidence accumulated per file: ClaudeRelationshipEvidence { file_session_id, first_ts, in_log_session_ids, source_version, first_parent_uuid, seen_uuids, has_resume_marker, resume_target_session_id, explicit_continuation_target_session_ids, explicit_fork_target_session_ids } (relationships.rs:23-38).
• file_session_id derivation honours only explicit signals (fileSessionId, then the transcript path basename) — never the opened path (relationships.rs:549-569); a transcript whose in-log sessionIds differ from its file name is itself relationship evidence.
• Codex: resume is currently handled implicitly by the token baseline (lib.rs:2260-2263); a resumed rollout's session_meta may carry the prior thread id — characterize on the fixture and record it.

relayhistory already stores parent_id (parentUuid) per event and has session_identity_correlations (crates/ai-hist-core/src/lib.rs:822) for remote/local pairs, so the substrate is close.

Scope

1. Extend session_relationships.relationship to delegated | materialized_local | continuation | fork | resume and add origin_session_id (the sourceSessionId a fork/continuation came from, when distinct from the parent) + evidence_ref usage: the uuid that linked the files, the marker text, or the explicit field name. Keep the PK (source, parent_session_id, relationship_uid); relationship_uid = "continuation:<child>" etc.
2. Claude parser: read forkSessionId, continuedFromSessionId, sourceSessionId, /resume//continue markers, and accumulate the per-file evidence struct above into a new session_continuity_evidence table (source, session_id, file_session_id, first_parent_uuid, first_ts, in_log_session_ids_json, resume_target, explicit_targets_json, source_version), keyed per transcript, so reconciliation can run incrementally during targeted hydration (one file at a time) rather than only during a full sync with every file in memory.
3. Reconciliation pass: after any Claude hydration/sync, resolve first_parent_uuid against session_events.event_uid/message_id across sessions and write continuation/fork rows; record RELATIONSHIP_CONTINUITY_UNRESOLVED diagnostics when the parent uuid is not (yet) indexed rather than guessing.
4. Codex: characterize resume on a real resumed rollout (fixture) and record a resume relationship when session_meta names a prior thread; otherwise document that Codex resume is not observable.
5. Traversal: session_tree / session_parents accept a relationship_kinds filter defaulting to delegated only (so existing tree semantics are unchanged) and can include continuity edges; getSessionRelationships returns them under a new continuity array. Bump SESSION_RELATIONSHIP_CONTRACT_VERSION (crates/ai-hist-core/src/relationships.rs:15); update contracts.ts, normalization.ts, MCP get_session_relationships, docs/architecture.md "Delegation topology" (rename to "Session topology").

Acceptance

• Corpus: fork-branch-a + fork-branch-b each get a fork row pointing at original-session; cross-file-parent gets a continuation row; resume-marker gets a resume row; explicit-line-relationships yields the explicit rows with evidence_ref naming the field.
• Hydrating fork-branch-a before original-session exists yields an RELATIONSHIP_CONTINUITY_UNRESOLVED diagnostic, and hydrating original-session afterwards resolves it without re-hydrating fork-branch-a.
• getSessionTree default output is byte-identical to before for delegation-only databases (existing relationships.test.ts unchanged).

Out of scope

Similarity-based linking (explicitly forbidden by docs/architecture.md: "title or prompt similarity is never used to infer one").