feat: per-project storage layer

[nonogv]

Summary

• Extends Storage with per-project paths for history, instructions, memories, and snapshots under projects/<slug>/, plus global context under global/.
• Adds project management (loadCurrentProject, saveCurrentProject, clearCurrentProject), scoped instructions/memories I/O, loadPromptContext, and loadProjectSnapshot / saveProjectSnapshot.
• One-time constructor migration copies legacy root history.json to projects/default/history.json when the per-project file does not yet exist.
• Exports ProjectInfo, ProjectSnapshot, and projectNameToSlug.

What was NOT touched

• src/server.ts — WebSocket handlers and history wiring (PR 3)
• src/agent/chat.ts — PromptContext export and buildSystemPrompt injection (PR 2); loadPromptContext uses a local matching interface until that PR lands
• ui/ — Context tab and project UI (PR 4)

Tradeoffs

• loadPromptContext defines a local PromptContext interface matching the shared contract rather than importing from chat.ts, so this PR stays storage-only and builds without PR 2. PR 2 can export the canonical type and this file can switch to import type.
• Project-scoped instructions/memories fall back to slug "default" when no current project is set (same as history).
• Legacy root history.json is copied, not deleted, so rollback remains possible.

Test plan

• npm test -- src/storage.test.ts — 34 tests pass (existing + new project/history/migration/context/snapshot coverage)
• npm run build — TypeScript and production bundle succeed
• After PR 2/3 merge: verify loadPromptContext import from chat.ts and server wiring

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