feat(coding-agent): add OpenViking memory backend

[pppobear]

Summary

• add OpenViking as a first-class memory backend with recall, retain, reflect, learn, automatic session capture, and memory:// document access
• isolate memory by workspace-derived peers while supporting explicit peers, cross-project recall, compatible server profiles, and safe cursor migration
• harden two-phase archive/extraction handling, cancellation, lifecycle transitions, live settings reconciliation, and secret display
• document the final OpenViking behavior and consolidate changelog entries under Unreleased

Why

OpenViking previously was not available through the coding agent's shared memory contract. This adds the backend while preserving workspace isolation, avoiding duplicated or lost transcript writes across failures and session transitions, and keeping parent/subagent memory state consistent.

Validation

• bun check
• 537 changed-area tests across 26 test files
• git diff --check