wip(rescue): OCP Phase 6c stream-json port + opus 4.8 (uncommitted /tmp work)

Rescue commit — preserves the subagent-produced Phase 6c port (claude -p →
stream-json + --system-prompt + NDJSON parser) and opus 4.8 model addition
done 2026-05-30 in a /tmp clone (volatile). NOT reviewed-for-merge; this is
preservation only on a WIP branch so a /tmp reboot does not lose the work.

Strategy context: OCP is being made the first-mover for TUI-mode (users are
on OCP). Phase 6c here is the DEFAULT-path foundation (stream-json = sdk-cli =
the "Max-user stable mode" fallback). TUI-mode (cc_entrypoint=cli subscription
bridge) lands on top as an opt-in. Re-review before merging to main.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

CHANGELOG.md

@@ -1,5 +1,13 @@
 # Changelog
 
+## Unreleased
+
+### Model — add claude-opus-4-8
+
+Add `claude-opus-4-8` as the newest Opus to `models.json` (index 0, newest first). Repoint `aliases.opus` from `claude-opus-4-7` to `claude-opus-4-8`. `claude-opus-4-7` remains in the list callable by literal id. `legacyAliases.claude-opus-4` left pointing at `claude-opus-4-7` (no change — legacy alias tracks the prior generation). README Available Models table and model-count references updated accordingly.
+
+---
+
 ## v3.16.4 — 2026-05-13
 
 ### Refactor — port-literal SPOT + CI guardrail