feat: create internal/stream/filestore.go for FileStore-backed strea...

[thruflo]

Summary

This PR implements a durable stream architecture that moves the Claude iteration loop from the developer's laptop to the Sprite VM. The core problem: if the laptop disconnects or sleeps, Claude stops iterating despite the Sprite VM remaining alive.

Solution: A new wisp-sprite binary runs directly on the Sprite, executing Claude iterations locally and persisting all events to a FileStore. TUI and web clients become equivalent stream subscribers that can disconnect and reconnect without losing output.

Architecture

Sprite VM                          Clients
 ┌──────────────────────┐     ┌─────────────────┐
 │  wisp-sprite         │     │  TUI / Web      │
 │  ├─ Loop (Claude)    │────▶│  StreamClient   │
 │  ├─ FileStore (NDJSON)│ SSE │  (reconnects)  │
 │  └─ HTTP :8374       │◀────│  POST /command  │
 └──────────────────────┘     └─────────────────┘

Key Changes

• cmd/wisp-sprite — New binary cross-compiled for Linux/amd64, runs iteration loop on Sprite
• internal/stream — FileStore for persistent NDJSON event storage; StreamClient for HTTP-based consumption with auto-reconnect
• internal/spriteloop — Loop logic, Claude executor, command processor, and HTTP server for Sprite-side execution
• internal/loop — Simplified to orchestration role; no longer runs iterations directly
• internal/tui / internal/server — Refactored to use StreamClient, enabling seamless disconnect/reconnect

Session Lifecycle

1. Start: Setup Sprite → upload wisp-sprite → start with nohup → TUI connects to stream
2. Disconnect: TUI exits cleanly; Sprite loop continues; FileStore persists events
3. Resume: TUI reconnects → catches up on missed events → resumes interaction

Tasks

This PR implements the following tasks:

• Create internal/stream package with shared message types
• Create internal/stream/filestore.go for FileStore-backed stream on Sprite
• Create internal/stream/client.go for stream client used by TUI/web
• Create internal/spriteloop package with loop logic for Sprite execution
• Create internal/spriteloop/claude.go for local Claude execution
• Create internal/spriteloop/commands.go for command processing
• Create internal/spriteloop/server.go for HTTP server on Sprite
• Create cmd/wisp-sprite binary entry point
• Add Makefile target for cross-compiling wisp-sprite for Linux/amd64
• Modify internal/cli/start.go to upload and start wisp-sprite binary
• Modify internal/cli/resume.go to reconnect to running wisp-sprite
• Refactor internal/tui to use stream client instead of direct loop channels
• Update internal/server/streams.go to work with stream client
• Simplify internal/loop/loop.go to orchestration-only role
• Write unit tests for internal/spriteloop package
• Write unit tests for internal/stream package
• Write integration test for disconnect/reconnect scenario
• Update AGENTS.md with new architecture documentation

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