[Rust port] relayburn-ingest: per-harness orchestration loops

[willwashburn]

Parent: #245

Context

The standalone modules of relayburn-ingest (pending_stamps, walk,
watch_loop, cursors) and the public verb surface landed in #245. The
per-harness orchestration loops — the bodies of ingest_claude_into,
ingest_codex_into, ingest_opencode_into, and the per-session fast-path
ingest_claude_session — are currently empty stubs that return an empty
report.

Scope

Fill in the orchestration helpers in crates/relayburn-ingest/src/ingest.rs:

• ingest_claude_into: walk ~/.claude/projects/<project>/, run
  parse_claude_session_incremental per file with cursor + lastUserText
  carry-over, append turns/content/events/relationships/toolResultEvents/userTurns,
  reconcile cross-file relationships at the end.
• ingest_codex_into: walk codex sessions, drive the resume/cumulative
  state machine, call resolve_pending_stamps_for_session for fresh
  ingests, derive sessionId from filename UUID or first-line hint.
• ingest_opencode_into: walk ses_*.json files, derive message dir
  mtime, drive parse_opencode_session_incremental with seenMessageIds.
• ingest_claude_session: replace the current stub with the real fast-path
  body — single parse_claude_session call, append, persist cursor at EOF.

Files

• crates/relayburn-ingest/src/ingest.rs — ingest_*_into helpers and
  the ingest_claude_session body.

Acceptance

• Each per-harness helper round-trips a fixture session through the
  full parse-and-append path.
• ingest_all integration test: drop a session file under each
  harness's session root, run ingest_all, assert turns appear in
  the ledger and cursors are persisted.
• ingest_claude_session test: writes one cursor at EOF such that a
  subsequent ingest_all skips the file.
• cargo test -p relayburn-ingest green.