Restore missing cmd/devvault entrypoint to unblock CI Build & Test

[Copilot]

The Build & Test (ubuntu-latest) job was failing because the E2E test builds ../cmd/devvault, but that command path no longer existed in the repository. This PR restores the expected CLI entrypoint path and prevents it from being accidentally ignored by Git patterns.

• Restore expected CLI binary entrypoint

  • Added cmd/devvault/main.go as the canonical executable entrypoint.
  • Wires the binary back to the existing CLI command tree in internal/cli.

• Scope ignore rules to root binaries only

  • Updated .gitignore entries from broad names (devvault, devvault-demo, etc.) to root-anchored paths (/devvault, /devvault-demo, etc.).
  • Prevents cmd/devvault/ source files from being treated as ignored artifacts.

package main

import "devvault/internal/cli"

func main() {
	cli.Execute()
}