Core: move the controller's editor-agnostic half into @orbit-code/core

[imshaikot]

Part of the Zed work (tracked in #9). First step, because a second host needs it and it touches nothing Zed-specific.

Why

apps/vscode/src/controller.ts holds two halves: the editor-agnostic one (checking every webview message, the snapshot on ready, a SessionProjector per conversation, attachable files and skills, the MCP action and resume-id checks, the generation counter) and the VS Code one (the panel, env.openExternal, notifications, FileActions over WorkspaceEdit). The monorepo skill ("Adding an editor") says to split them before building another host; apps/server (#2) is that host.

What

• Move the editor-agnostic half into packages/core (e.g. core/src/hostController.ts), behind a small interface the editor implements: send a message to the UI, open an external URL, notify (permission while hidden, busy, unavailable), and the file operations FileActions does today (git state, read/write against a revision, rename, delete, open in the editor, diff).
• apps/vscode keeps OrbitController as the VS Code implementation of that interface plus the panel and commands. fileRequest() (what activate() returns for smoke) keeps its shape.
• hostSim.ts is unaffected; check-wiring.mjs may need its paths updated (it reads controller.ts for the protocol and log lines: see the note in CLAUDE.md "Smoke test dependencies", the log lines must not be reworded).
• CLAUDE.md Host table and the monorepo skill's "Adding an editor" table: point at the new module.

Checks

yarn typecheck && yarn boundaries && yarn build, node .claude/skills/orbit-extension/scripts/check-wiring.mjs, yarn harness (read the report), yarn smoke.