Transport abstraction: decouple tools from local ADB binary

[ArtemisAI]

Problem

src/adb/exec.ts:41 hardcodes execFile("adb", …) — every tool, plugin, and flow assumes the adb binary lives on the same host as pi-droid. This blocks three topologies:

• Remote: agent on laptop, phone USB-connected to a different host (e.g., today's Brain setup) — currently requires running pi-droid on the remote host
• On-device (Termux): pi-droid as a worker inside the phone — no adb needed, shell is direct
• WiFi ADB from anywhere: works today but still needs adb local to pi-droid

Proposal

Introduce a Transport interface that every I/O routes through. adb() / adbShell() become thin wrappers over Transport.shell() / Transport.adb(), defaulting to a back-compat LocalAdbTransport.

interface Transport {
  readonly capabilities: { adb: boolean; portForward: boolean; hostFs: "local"|"remote"|"android"; screenMirror: "scrcpy"|"ws-scrcpy"|"native"|"none" };
  shell(cmd, opts): Promise<string>;
  adb(args, opts): Promise<string>;   // throws UnsupportedByTransport on on-device
  forward(localPort, remote): Promise<void>;
  push/pull(...)
  screenshot(): Promise<Buffer>;
}

Implementations:

• LocalAdbTransport — today's behavior (Phase 1)
• RemoteAdbTransport — wraps SSH: ssh <host> adb -s <serial> … (Phase 3)
• OnDeviceTransport — Termux; adb() throws, shell() goes direct (Phase 4)

Plugin impact

Direct execAsync("adb", …) calls (e.g. private/flows/ollama-setup.ts:167 for the CDP port forward) migrate to transport.forward(9222, "localabstract:chrome_devtools_remote") — then works unchanged on remote transport and no-ops cleanly on on-device.

Non-goals

• Cross-topology failover mid-session
• Emulator support (tracked separately)

Design doc

Full design: docs/CONNECTIVITY.md — covers the three topologies, migration phases, and back-compat story.

Depends on / blocks

• Blocks: Workspace layout: move runtime data out of the repo into ~/.pi-droid/ #11, On-device worker: run pi-droid inside the phone (Termux) #12
• Related: Issue 1 (standalone pidroid CLI, from ISSUES-TO-FILE.md)