fix(cli): emit single JSON document from session show with sidechains

[zawakin]

Problem

agtrace session show <id> --format json produced invalid JSON when the session contained sidechains (subagent streams). Each stream was rendered as a separate JSON document, with text separators printed between them:

{ ...main stream... }

────────────────────────────────────────
Additional Stream: sidechain:a1da8e6a… (spawned by Turn #2, Step #2)

{ ...sidechain stream... }

Root cause: handlers/session_show.rs looped over streams calling ctx.render() once per stream and println!-ing separators unconditionally — bypassing the format abstraction entirely.

Fix (schema-level redesign)

One session show invocation now produces exactly one view model → one document, in every format:

• New schema: SessionDetailViewModel { session, streams[] } replaces the per-stream SessionAnalysisViewModel/SessionHeader. Session-scoped metadata (id, provider, project, log files) appears once; each stream carries its own stream_id, spawned_by (raw 0-based indices), status, context_summary, and turns.
• Presenter (present_session_detail) takes all assembled streams, orders them Main-first, and attaches subagent children to the main stream.
• View (SessionDetailView) owns the text-mode stream separators/headings that the handler previously printed — the handler no longer prints anything itself.
• agtrace-sdk now re-exports SpawnContext so the CLI can consume it through the SDK boundary.

Verification

• Regression test: 3-stream view model serializes to a single parseable JSON document.
• E2E on the exact session that produced the broken dump (main + 3 sidechains): output is now one valid JSON document with content.streams[4], each sidechain carrying structured spawned_by. Text mode still renders per-stream sections with separators.
• cargo test: all pass. Clippy is clean on 1.90.0 (the 6 pre-existing warnings visible on newer local toolchains also exist on main).

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