Add direct OpenCode stream ingest

[willwashburn]

Closes #92

Summary:

• Add a reader-level OpenCode stream ingestor that normalizes SSE events into turns, content, user turns, relationships, and tool-result events for sessions observed from session.created.
• Wire burn watch --opencode-stream to append stream-derived records, persist the Last-Event-ID cursor, and keep file polling as the fallback/drift path.
• Add conservative per-tool usage attribution for OpenCode tool-result events and content sidecar dedupe for stream/file overlap.
• Keep already-running sessions on file fallback to avoid stream event-index drift.

Tests:

• pnpm run build
• pnpm run test
• node --test --test-reporter=spec packages/reader/dist/opencode-stream.test.js packages/reader/dist/opencode.test.js packages/cli/dist/opencode-stream.test.js packages/ledger/dist/content.test.js

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