feat: server WebSocket layer for per-project context

[nonogv]

Summary

• Implements the full backend stack for per-project context layering: storage (PR1), chat prompt injection (PR2), and server WebSocket handlers (PR3) on a single self-contained branch for review.
• Storage: project-scoped history, instructions, memories, snapshots; legacy history.json migration; loadPromptContext() helper.
• Chat: PromptContext interface and buildSystemPrompt context sections appended at the end.
• Server: historyRef pattern for project switching; new WS messages (project, context, context_saved, project_stale); handlers for get/set/clear_project, get_context, save_instructions, save_memories, refresh_project_memories.

What was NOT touched

• ui/ — frontend (PR4) is out of scope for this PR.

Tradeoffs

• This branch includes PR1+PR2 changes inline so it compiles and is reviewable before those PRs merge. It should be rebased after PR1 and PR2 land in main to drop duplicate commits.
• save_project_snapshot is only called on refresh_project_memories, not on manual save_memories (per contract).

Test plan

• npm test — 97 tests pass
• npm run build — TypeScript + extension + UI build succeed
• Manual: connect UI, verify project + context frames on connect
• Manual: switch project, confirm history isolation
• Manual: refresh project memories streams and saves snapshot

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