refactor: replace global singletons with AgentContext instance

[hewliyang]

Summary

Replaces the scattered module-level singleton pattern (setCustomCommands(), configureNamespace(), getVfs(), getBash(), etc.) with a single AgentContext class that AgentRuntime owns and threads to all consumers.

What changed

New: AgentContext (packages/sdk/src/context.ts)

• Owns VFS (InMemoryFs), bash (Bash), and StorageNamespace
• Provides file operations (readFile, writeFile, deleteFile, fileExists, listUploads)
• Provides VFS lifecycle (reset, restoreVfs, snapshotVfs)
• Constructed by AgentRuntime, passed explicitly to tools and storage functions

Tools → factories

• bashTool → createBashTool(ctx: AgentContext)
• readTool → createReadTool(ctx: AgentContext)
• evalOfficeJsTool → createEvalOfficeJsTool(ctx) (Excel, PPT, Word variants)
• getOoxmlTool → createGetOoxmlTool(ctx) (Word)
• App tool indices export createExcelTools(ctx), createPptTools(ctx), createWordTools(ctx)

Adapter pattern update

• AppAdapter.tools accepts AgentTool[] | ((ctx: AgentContext) => AgentTool[]) — factory or static array
• AppAdapter.customCommands now receives (ns: StorageNamespace) => CustomCommandsResult from the runtime
• No more hardcoded prefix strings ("openexcel", "openppt", "openword") in custom command files

localStorage helpers take StorageNamespace

• loadSavedConfig(ns), saveConfig(ns, config)
• loadWebConfig(ns), saveWebConfig(ns, config)
• loadOAuthCredentials(ns, provider), saveOAuthCredentials(ns, provider, creds), removeOAuthCredentials(ns, provider)
• Settings panel uses chat.context.namespace instead of a raw prefix string

Storage functions take StorageNamespace

• All storage/db.ts functions (createSession, saveSession, listSessions, etc.) take ns as first param
• storage/namespace.ts reduced to a type re-export

Deleted globals

• setCustomCommands(), setStaticFiles(), setSkillFiles(), resetVfs(), getVfs(), getBash()
• configureNamespace(), getNamespace()
• readFile(), writeFile(), readFileBuffer(), fileExists(), deleteFile(), listUploads(), snapshotVfs(), restoreVfs() (all now on AgentContext)
• vfs/index.ts gutted to pure utilities only (getFileType, detectImageMimeType, toBase64)

Tests

• All tests create isolated AgentContext instances — no shared global state
• No more resetVfs() / setStaticFiles({}) ceremony in beforeEach/afterEach

Bridge

• Bridge client handles tools as array or factory
• VFS pass-through from app.svelte removed (bridge VFS support to be re-wired separately)

Why

• No hidden coupling — every function declares what it needs
• No ordering bugs — context is fully constructed before use
• Testable — spin up isolated instances, no global resets
• Self-documenting — chat.context.namespace instead of magic prefix strings