Ledger: persist UserTurnRecord and forward through ingest

[willwashburn]

Context

PR #74 landed UserTurnRecord / UserTurnBlock in @relayburn/reader and emits them from the Claude parser, but stopped short of persistence. From the PR's Deferred section:

Persisting UserTurnRecord to the ledger. Currently in-memory parser output only; persistence (and any schema v bump) is a separate decision driven by the first consumer (burn waste).

Today the parser returns userTurns: UserTurnRecord[] (packages/reader/src/claude.ts:121-126, packages/reader/src/claude.ts:283, packages/reader/src/claude.ts:1149-1156) but the CLI ingest path drops them on the floor — no reference to userTurns exists in packages/cli/src/ingest.ts or packages/ledger/src/. The ledger schema (packages/ledger/src/schema.ts:38) defines LedgerLine = TurnLine | StampLine | CompactionLine; there's no UserTurnLine. The writer (packages/ledger/src/writer.ts:45-69 for appendTurns, :71-94 for appendCompactions) has no equivalent for user turns.

Without persistence, the per-tool-call cost attribution consumer (the original goal of #2) can't read user-turn block sizes back out of the ledger; it can only see them when re-parsing source session files at query time, which defeats the point of an incremental, append-only ledger.

Proposal

Wire UserTurnRecord end-to-end:

1. Schema: add a UserTurnLine variant alongside TurnLine / StampLine / CompactionLine in packages/ledger/src/schema.ts. Add isUserTurnLine predicate. Update LedgerLine union.
2. Writer: add appendUserTurns(records: UserTurnRecord[]) in packages/ledger/src/writer.ts, modeled on appendCompactions — dedup via index-sidecar.ts using a userTurnIdHash (hash of sessionId + userUuid), append-only, holds the same ledger lock.
3. Index sidecar: add userTurnIdHash next to turnIdHash / compactionIdHash in packages/ledger/src/index-sidecar.ts. Reuse the existing id namespace.
4. Reader: extend packages/ledger/src/reader.ts query API so consumers can fetch user turns alongside turns (e.g. queryUserTurns(q: Query) or include them in an enriched query result keyed by sessionId).
5. Ingest: in packages/cli/src/ingest.ts, forward the userTurns returned by parseClaudeSession{,Incremental} into appendUserTurns. Future Codex/OpenCode parsers (issues Reader: populate UserTurnRecord for Codex sessions #81, Reader: populate UserTurnRecord for OpenCode sessions #86) plug into the same call site.
6. Schema version: this is additive — existing readers already destructure specific line kinds and ignore unknown ones — so no v bump on TurnRecord. The new UserTurnLine itself carries v: 1.
7. AGENTS.md / package CHANGELOGs: note the new line kind in @relayburn/ledger and the ingest forwarding.

Acceptance criteria

• LedgerLine union includes UserTurnLine and isUserTurnLine predicate is exported.
• appendUserTurns exists, dedupes by stable id, and holds the ledger lock.
• Running burn ingest against a Claude session produces user-turn lines in the ledger file (grep '\"kind\":\"user-turn\"' \$RELAYBURN_HOME/ledger.jsonl returns rows).
• Re-running ingest is a no-op for already-persisted user turns (dedup test).
• Ledger reader exposes user turns either via a dedicated query function or as an enriched-turn association keyed by precedingMessageId / followingMessageId.
• Existing ledger consumers (turns / stamps / compactions) keep working unchanged — older readers gracefully ignore the new line kind.
• Tests: writer dedup, reader round-trip, ingest forward.

Out of scope

• Codex / OpenCode parsers populating userTurns — issues Reader: populate UserTurnRecord for Codex sessions #81 / Reader: populate UserTurnRecord for OpenCode sessions #86.
• Per-tool-call cost attribution consumer reading from the new ledger lines — separate follow-up.
• Backfilling user turns into existing ledgers (burn rebuild) — note in CHANGELOG; can be a separate issue if needed.
• Tokenizer accuracy upgrade — separate follow-up.

Refs

• Spun out of Reader: capture per-user-turn block sizes for Claude, Codex, and OpenCode sessions (#2, #81, #86) #74 (Reader: preserve user-turn block sizes to unlock per-tool-call cost attribution #2 follow-up)
• Related: Reader: preserve user-turn block sizes to unlock per-tool-call cost attribution #2 (parent), Reader: populate UserTurnRecord for Codex sessions #81, Reader: populate UserTurnRecord for OpenCode sessions #86